Differential voltage follower

ABSTRACT

A two-step analog-to-digital converter and BiCMOS fabrication method. The fabrication method provides pseudosubstrate isolation of digital CMOS devices from the analog devices. The converter uses NPN current switching in a flash analog-to-digital converter and in a digital-to-analog converter for low noise operation. CMOS digital error correction and BiCMOS output drivers provide high packing density plus large output load handling. Timing control aggregates switching events and puts them into intervals when noise sensitive operations are inactive. The fabrication method uses a thin epitaxial layer with limited thermal processing to provide NPN and PNP devices with large breakdown and Early voltages. Laser trimmed resistors provide small long term drift due to dopant stabilization in underlying BPSG and low hydrogen nitride passivation.

CROSS-REFERENCE TO RELATED APPLICATIONS

The following U.S. patent applications are assigned to the assignee ofthis application and disclose related subject matter: Ser. No. 785,325,filed Oct. 30, 1991 and Ser. No. 785,400, filed Oct. 30, 1991.

FIELD OF THE INVENTION

The present invention relates to electronic semiconductor devices andmethods of fabrication, and, more particularly, to semiconductor devicesuseful for conversion between analog and digital signals and fabricationmethods integrating both bipolar and field effect devices.

BACKGROUND AND SUMMARY OF THE INVENTION

Digital processing and transmission of electrical signals has becomecommonplace even for basically analog information. Examples range fromhandheld digital voltmeters to the transition beginning in the 1960s ofthe public long distance telephone network from analog transmission topulse code modulation (PCM) digital transmission. Application of digitalmethods to analog information requires an analog-to-digital (A/D)conversion, and the linearity, resolution, and speed of such conversiondepends upon the application. For example, digital voltmeters usuallycall for A/D conversion with good linearity and resolution (18-bits) butwhich may be slow (1 Hz); whereas, video applications demand high speed(30 million samples and conversions per second) but tolerate lowresolution (8-bits) and poor linearity. Intermediate requirements of12-bit resolution, good linearity, and 3 Msps (million samples persecond) speed appear in applications such as medical imaging withultrasound, robotic control, high speed data acquisition, processcontrol, radar signal analysis, disk drive head control, vibrationanalysis, waveform spectral analysis, and so forth. Multichannelinformation acquisition with arrays of A/D converters leads to anotherrequirement: small aperture jitter so that synchronism of the channelscan be maintained.

Well known types of A/D converters include the successive approximationsconverter which produces a digital output by a succession oftrial-and-error steps using a digital-to-analog converter (DAC) and theflash converter which compares an input signal to multiple referencelevels simultaneously and outputs a digital version of the closestreference level in a single step. The successive approximationsconverter provides high resolution and linearity but with low conversionspeed, and the flash supplies high speed at the cost of resolution andlinearity. Note that a flash converter with n-bit resolution typicallyhas a voltage divider with 2^(n) taps and 2^(n) comparators, and thisbecomes unwieldy for high resolution. See, however, copending U.S.patent application Ser. No. 696,241, filed May 6, 1991 and assigned tothe assignee of the present application. A compromise between these twotypes is the two-step flash A/D converter which uses a first coarseflash conversion to find the most significant bits and then reconstructsan analog signal from first flash output and subtracts this from theinput signal to create an error signal from which a second flashconversion finds the least significant bits. Generally see Grebene,Bipolar and MOS Analog Integrated Circuit Design (Wiley-Interscience1984), page 871. It is desirable that A/D converters combine stillhigher speed and resolution with lower noise.

Methods of fabrication used for various semiconductor devices includethe combination of bipolar transistors with CMOS transistors (BiCMOS),with analog portions of the integrated circuit using mainly bipolartransistors for their low noise and digital portions using mainly CMOStransistors for their high packing density. See for example R. Haken etal, "BiCMOS Processes for Digital and Analog Devices," SemiconductorInternational 96 (June 1989). However, improved BicMOS fabricationmethods are needed to achieve higher speed and resolution with lowernoise on a monolithic circuit.

The present invention provides a monolithic two-step flash A/D converterwith high speed and resolution and a BiCMOS method of fabricationapplicable to such converters and other integrated circuits.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described with reference to theaccompanying drawings which are schematic for clarity,

FIGS. 1-2 illustrate applications of a preferred embodimentanalog-to-digital converter;

FIG. 3 is a functional block circuit diagram of the preferredembodiment;

FIGS. 4-5 are flow and timing diagrams for the operation of thepreferred embodiment;

FIGS. 6, 7a-7e and 8 show aspects of the sample and hold of thepreferred embodiment;

FIGS. 9, 10, 11-13, 14a, 14b and 15-22 show aspects of the flashconverter or the preferred embodiment;

FIGS. 23a-23d, 24a-24d, 25, 26, 27a-27c and 28-30 show aspects of thedigital-to-analog converter of the preferred embodiment;

FIGS. 31-32, 33a-33d, 34-36 and 37a-37c show aspects of the erroramplifier of the preferred embodiment;

FIGS. 38, 39 show aspects of the error correction of the preferredembodiment;

FIG. 40-44 show aspects of the output buffer of the preferredembodiment;

FIGS. 45, 46a-46b and 47-49 show aspects of the timing controller of thepreferred embodiment;

FIGS. 50-51 show aspects of the power up reset of the preferredembodiment;

FIGS. 52a-52b and 53-57 show aspects of the reference voltage generatorof the preferred embodiment;

FIGS. 58a-58d are layouts for some preferred embodiment devices;

FIGS. 59a-59h are profiles for some preferred embodiment devices;

FIGS. 60-80 are cross-sectional elevation views of steps of a preferredembodiment method of fabrication;

FIGS. 81-85 show aspects of the ESD protection of the preferredembodiment;

FIGS. 86-87 show aspects of the isolation structure of the preferredembodiment;

FIGS. 88-96 show aspects of the alternative embodiments; and

FIG. 97 illustrates a time-temperature trade-off.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates an ultrasound analysis system 100 whichincludes a sound generator 102, sound detector 104, first preferredembodiment analog-to-digital converter 106, digital signal processor108, and video display 110. System 100 generates high frequency (100KHz) sound waves that penetrate object 120, and these waves reflect frominterior structures of object 120 to be detected by detector 104.Converter 106 converts the detected analog signal to a digital form forsignal processing by DSP 108, and video display 110 presents the resultson a CRT. Mechanically scanning the sound generator 102 and detector 104over the surface of object 120 provide reflection information toreconstruct an image of the interior structure. Use of system 100 forhuman medical diagnosis or analysis demands relatively high speedoperation for patient convenience and relatively high resolution forimage reconstruction.

Converter 106 is a 12-bit, subranging (half-flash or two-step) converterwith digital error correction which samples an analog input in the rangeof -2.5 volts to +2.5 volts at a sampling rate of 3 Msps (millionsamples per second) and with an input bandwidth of 30 MHz. 12-bitresolution implies that the least significant bit of output correspondsto a 1.22 mV input interval. An input bandwidth of 30 MHz means thatconverter 106 can track video signals and that an array of converters106 with sequential clocking can provide video digitization; see FIG. 2which shows n converters 106 clocked by sequential commands CONV1,CONV2, . . . CONVn. This array gives an effective sampling rate of 3 nMHz.

Converter 106 operates over a temperature range of -55 C. to +125 C.with integral and differential linearity error and full scale error allabout or less than 1 bit. Converter 106 uses a combination of bipolarand CMOS (BiCMOS) devices together with polysilicon-polysiliconcapacitors and nickel-chromium thin film resistors plus laser trimming.Most CMOS gate lengths are about 1 μm and NPN emitters typically areabout 2 μm by 3 μm with devices scaled to provide larger emitter areas.Also, matched devices may be split and laid out in symmetricalarrangements to help thermal balance and insensitivity.

Converter Overview

FIG. 3 is a functional block diagram of the first preferred embodimentconverter, indicated generally by reference numeral 300, which includesanalog signal input terminal 302, sample and hold block 304, 7-bit flashanalog-to-digital converter block 306, most-significant-bits (MSB) latch308, 7-bit digital-to-analog converter (DAC) block 310 (the DAC istrimmed to more than 14-bit accuracy), error amplifier 312,least-significant-bits (LSB) latch 314, subtractor 316, error correctionblock 318, output buffer 320, output port 322, overflow/underflow blockOF/UF 324, voltage reference block 326 with output terminal 328, timingcontroller and oscillator block 330, conversion command input terminal332, and analog switch 334. Converter 300 is a two-step subranginganalog-to-digital converter which uses the same 7-bit flash converterfor both the MSB and the LSB conversions. Correction of system errorsmakes use of MSB and LSB overlap. The 12-bit output uses a two'scomplement representation of negative numbers, so an input of 0 voltsleads to an output of 1000 0000 0000, an input of -1.22 mV gives anoutput of 0111 1111 1111, and an input of - 2.5 volts yields 0000 00000000. An input of +1.22 mV gives an output of 1000 0000 0001, and aninput of +2.5 volts yields 1111 1111 1111.

Operation Summary

FIG. 4 is a flow diagram for a conversion by converter 300 and FIG. 5 isa timing diagram (in nanoseconds) for the conversion flow whichbasically proceeds as follows. A falling edge of the convert command(CONV) input at terminal 332 begins the conversion process; see thebottom panel of FIG. 5. Just prior to the CONV command, sample and hold304 was tracking (following) the input V_(in) (t) at terminal 302,analog switch 334 was connecting the output of sample and hold 304 toflash converter 306, the comparators and encoder of flash converter 306were following the sample and hold 304 output (which ideally is V_(in)(t)) but without latching, DAC 310 was holding at a 0 volt output due toa fixed input, and error amplifier 312 was clamped to a 0 volt output.The CONV command at time t₀ switches sample and hold 304 into the holdmode and with a fixed output equal to V_(in) (t₀). This switchingrequires a settling time of about 30 nanoseconds (nsec) due to chargeinjection by the switch; see the HLDSTTL pulse in the second panel fromthe bottom of FIG. 5. For simplicity, V_(in) (t₀) will be called V_(in).At the end of the HLDSTTL pulse, the first flash convert clock FLASH1rises to latch the comparators of flash converter 306 which have beenfollowing the essentially constant output V_(in) of sample and hold 304.The comparators ideally are outputting a quantization of V_(in) to theencoder which has been encoding this quantization as a 7-bit number; seethe FLASH1 pulse in the third from bottom panel of FIG. 5. After about28 nsec to allow settling by the latching circuitry of flash converter306, the FLASH1 pulse falls low to store the 7-bit output in MSB latch308 (not shown in FIG. 4 but incorporated in Digital Subtractor andError Correction). This 7-bit output is the binary encoding of thequantized version of the input signal V_(in) with quantization levelsseparated by about 39 mV. Because the final 12-bit output of converter300 will be a binary encoding of the quantized version of V_(in) withquantization levels separated by 1.22 mV (39 mV divided by 32), this7-bit output contributes only to the seven most significant bits of thefinal 12-bit output. Note that an output of 0000000 from flash converter306 corresponds to an input signal of about -2.5 volts, whereas an inputsignal of about 0 volts will lead to an output of 1000000 and an inputof about +2.5 volts will yield an output of 1111111.

Next, the rising edge of the 80 nsec DAC settling pulse (DACSTTL pulsein the fourth from bottom panel of FIG. 5) performs three tasks: (1) itputs the 7-bit output of flash converter 306 stored in MSB latch 308into DAC 310, which reconstructs the quantization of V_(in) ⁻ from the 7bits, this reconstruction is denoted V_(rq) below, (2) it puts theoutput of flash converter 306 into substractor 316 which adds a fixed7-bit code to compensate for the bipolar mode of operation and errorcorrection, and (3) it switches analog switch 334 to connect the outputof error amplifier 312 to the input of flash converter 306. Then the DAC310 output begins slewing towards its final value, V_(rq), and feeds aninput of error amplifier 312 which, however, remains clamped for about10 nsec to avoid noise and saturation problems. During the remaining 70nsec of the DACSTTL pulse, DAC 310 settles to its final output V_(rq)and error amplifier 312 amplifies the difference between V_(in) andV_(rq) by a factor of 32. That is, error amplifier 312 amplifies thequantization error by 32; see the left middle portion of FIG. 4.

The second step conversion begins at the end of the DACSTTL pulse: flashconverter 306 has been following the output of error amplifier 312 whichhas been settling to the amplified quantization error, and the risingedge of the second flash convert pulse (FLASH2 in the fifth from bottompanel of FIG. 5) latches the comparators of flash convertor 306. Thefalling edge of FLASH2 28 nsec later stores the encoded quantizedversion of the amplified quantization error in LSB latch 314, whichfeeds the most significant two bits to error correction block 318. Dueto the amplification factor being only 32, rather than 128 as 7-bitconversion would suggest, the second conversion's most significant bitsoverlap the first conversion's least significant bits.

If the components of convertor 300 were errorless, then the overallconversion would amount to the following. The first flash conversioneffectively decomposes V_(in) as

    V.sub.in =V.sub.q +(V.sub.in -V.sub.q)

where V_(q) is the quantized version of V_(in) with quantization levelsseparated by about 39 mV and (V_(in) -V_(q)) is the first quantizationerror. The 7-bit output in MSB Latch encodes V_(q). DAC 310 errorlesslyreconstructs V_(q) from the 7 bits in MSB Latch 308; that is, V_(rq)equals V_(q). Next, the second flash conversion effectively decomposesthe amplified first quantization error 32(V_(in) -V_(q)) as

    32(V.sub.in -V.sub.q)=W.sub.q +[32(V.sub.in -V.sub.q)-W.sub.q ]

where W_(q) is the quantized version of 32(V_(in) -V_(q)). Again thequantization levels are separated by about 39 mV and [32(V_(in)-V_(q))-W_(q) ] is the second quantization error. The 7-bit output inLSB Latch encodes W_(q). So the final quantized output is V_(q) +W_(q)/32 with roughly V_(q) generating the most significant bits and W_(q)/32 the least significant bits. Thus the combined effect of both flashconversions is to decompose V_(in) as

    V.sub.in =V.sub.q +W.sub.q /32+[32(V.sub.in -V.sub.q)-W.sub.q ]/32

That is, the overall quantization error equals the second quantizationerror divided by 32; so the overall quantization error is at most 39mV/32 which equals 1.22 mV.

Error correction block 318 corrects any dynamic error (within tolerance)caused by the limited linearity accuracy of flash converter 306 duringthe first conversion step; the two most significant bits of the secondconversion overlap the two least significant bits of the firstconversion and provide the basis for the correction. Error correctionblock 318 provides the seven most significant bits and LSB latch 314 thefive least significant bits to 12-bit output buffer 320 which makes thebits available at output port 322. Error correction and output buffer320 loading consume about 20 nsec; see the LOADOP pulse in the sixthfrom bottom panel of FIG. 5. This completes the overall conversion; andif CONV remains low, another sampling and conversion begins. The seventhfrom bottom panel of FIG. 5 shows the ACQUIRE pulse which activatessample and hold 304 to acquire another sample, and the eighth frombottom panel (the top panel) of FIG. 5 shows the End of Conversion pulseEOC. The settling time for sample and hold 304 after switching from holdmode to sample mode is about 100 nsec and uses both the 80 nsec ACQUIREpulse and the 20 nsec EOC pulse. The righthand portion of the secondfrom bottom panel of FIG. 5 indicates the HLDSTTL pulse of the nextconversion.

The analog signal input range is 5 volts (-2.5 volts to +2.5 volts), sothe quantization, 7-bit encoding, and subsequent analog reconstructionof input signal V_(in) will ideally yield a quantized approximationV_(rq) with level spacings of 39.0625 mV and such that the approximationonly differs from the input signal by at most one-half of a levelspacing (19.53125 mV). Hence the difference, V_(in) -V_(rq), afteramplification by a factor of 32 in error amplifier 312, will ideallyfall in the range of -625 mV to +625 mV and thus not exceed one quarterof the input range of flash converter 306. Therefore, the output of thesecond pass through flash converter 306 should be seven bits with thethree most significant bits being either 011 or 100 for negative orpositive inputs, respectively. Consequently, the two most significantbits of the second pass overlap the two least significant bits of thefirst pass through flash converter 306, and this implies a 12-bitoverall output rather than a 14-bit output as would have been guessedfrom the two 7-bit conversions. Discussion of error correction block 318below details this overlapping of bits and also leads tooverflow/underflow block 324 which indicates an original input out ofthe -2.5 to +2.5 volts range.

Converter 300 has the following features: the timing pulses driving theoperation do not overlap; only one function runs at a time, whichlessens noise coupling; the sample and hold control provides aperturedelay of less than 20 nanoseconds and aperture jitter of less than 25picoseconds; clock signals driving flash converter 306 are translated tobipolar levels with a swing of 0.7 volts (V_(be)) and lessen switchingnoise; subtractor 316 completes its operation prior to the activation oferror amplifier 312 to lessen noise problems and avoid overdrive; theswitching delay in activation of error amplifier 312 permits a settlingof the DAC 310 output; and the output buffer 320 turns on its driverssequentially to lessen ground bounce. The small aperture jitter permitsthe parallel configuration of converters, as illustrated in FIG. 2.

Converter 300 uses separate digital and analog power supplies anddigital and analog grounds. The power supplies Vcc and Vdd are at +5volts and Vee and Vss are at -5 volts with analog bipolar and CMOSdevices operating between +5 and -5 volts but with the digital CMOSdevices operating between +5 volts and ground.

FIGS. 6-57 illustrate the elements of converter 300 in greater detail,including elements only implicitly shown in FIG. 3; and the accompanyingdescription follows the same order as the preceding overview.

Sample and Hold

FIGS. 6-7f schematically show circuitry of sample and hold block 304with FIG. 6 providing a functional block diagram and FIGS. 7a-f aschematic circuit diagram. FIG. 8 shows settling from a 2.5 volt inputstep function. As seen in FIG. 6, sample and hold 304 includesdifferential amplifier 602, differential amplifier 604, and capacitor606 arranged as a closed-loop integrating type sample and hold circuit.Timing controller block 330 controls switch 608 through buffer 610.

In the sample mode, switch 608 connects the output of amplifier 602 tothe inverting input of amplifier 604 which charges or dischargescapacitor 606 so that the output Vout tracks the input V_(in) atterminal 302. During hold mode switch 608 connects the output ofamplifier 602 to ground to prevent saturation, and amplifier 604 holdsthe charge on capacitor 606 and also drives the bipolar input of erroramplifier 312 and, when analog switch 334 is thrown, the bipolar inputof flash converter 306.

NPN devices are used in the input amplifier where device matching, highspeed, and large transconductance are needed. MOS transistors are usedin the sample and hold switch where their low off-state leakage, fastswitching speed, and charge injection compensation ensure low pedestalerror and fast hold mode settling. The high input impedance of MOStransistors is utilized in the input stage of the output amplifier. Thehigh input impedance provides a very low droop rate. The high speedcharacteristics of the bipolar transistors are utilized in the rest ofthe outpu amplifier (gain and output stages) to achieve a largebandwidth which translates into low acquisition times.

FIGS. 7a-f show amplifier 602 as a high output impedancetransconductance amplifier. The inputs 701-702 connect to a modifiedDarlington differential pair 703-704 with emitter degeneration resistor706 for improved slew rate; the inputs (which are V_(in) and Vout) areto be in the range of -2.5 to +2.5 volts and the rails are at +5 voltsand -5 volts. The outputs of the differential pair 703-704 connect tothe sources of PMOS cascode devices 707-708 which replace PNP devicesand provide a high frequency level shift function and drive the Wilsoncurrent mirror made of NPNs 710-715. The single-ended output ofamplifier 602 at node 718 connects to sample and hold switch 608 whichconsists of a pair of CMOS transmission gates 720-721, gate 720 connectsoutput node 718 to ground and gate 721 connects output node 718 toinverting input 731 of amplifier 604 and capacitor 606. The CMOStransmission gate switch includes charge cancelling devices to reducecharge injection error and leakage current. The switch control signal(called IRQ below) from block 330 enters node 730 and directly driveslevel translator 725 to switch gate 721 but is delayed by inverter chain727 for driving level translator 724 to switch gate 720. Hence,switching from sample mode to hold mode has a few nsec gap between thedisconnection of the output of amplifier 602 from the inverting input ofamplifier 604 to the connection of the output to ground. This gap avoidsinjecting charge from the switching to ground into holding capacitor 606and thus lessens pedestal error.

Amplifier 604 is a two gain stage amplifier with a large PMOSsource-coupled pair used as an input differential pair 731-732 toprovide high input impedance, low noise, and no dc gate current andusing a NPN current mirror load 734. The single-ended output of the PMOSpair 731-732 drives an all-NPN output stage 736. FIGS. 7a,e also showstart up circuit 740, bias circuit 742 for amplifier 602, and biascircuit 744 for amplifier 604; the use of separate bias circuits limitsnoise and talkback.

Capacitor 606 has 15 pF capacitance and is made of two layers ofpolysilicon separated by a grown oxide of 900 Å thickness for lowleakage. Both amplifier 602 and amplifier 604 are made of a combinationof CMOS and NPN devices, which permits the fast, high gain of amplifier602 (input impedance of about 20 Mohms) and the low leakage input ofamplifier 604 during the hold mode. The high gain plus the grounding ofamplifier 602 during hold mode to prevent saturation (the input atV_(in) keeps changing whereas Vout holds, so the differential input canbecome large permits an acquisition time of less than 100 nsec for 0.01%error; that is, after switching to sample mode Vout tracks within 0.5 mVof V_(in) within 100 nsec. See FIG. 8, which illustrates the extremecase of V_(out) initially at 0 volts and V_(in) at +2.5 volts. The drooprate is less than 1 mV/μsec.

FIGS. 7g-l illustrate an alternate embodiment of the sample and hold 304using PNP transistors. The PMOS cascode devices 707 and 708 are replacedwith PNP bipolar transistors 707A and 708A to exploit their superiorfrequency response. The greater transconductance of the PNP transistorpresents a lower impedance to the collectors of the input transistors703 and 704, which reduces the parasitic time constant and improvesacquisition time. A push-pull type output stage is made possible by theaddition of the complementary PNP transistors 750 and 751. This type ofoutput stage is capable of driving lower impedance loads. For a givenload, the addition of the PNP will reduce phase shift in the outputstage and allow a greater overall bandwidth.

The following table compares the improved specifications of thepreferred embodiment sample and hold amplifiers to that presentlyavailable.

    ______________________________________                                        Parameter    Prior Art FIGS. 7 a-f                                                                              FIGS. 7 g-l                                 ______________________________________                                        Input Range  ±10 V  ±2.5 V  ±2.5 V                                   Input Resistance                                                                           15 Meg    100K Ohms  500K Ohms                                   Input Capacitance                                                                          <5 Pf     <5 Pf      <5 Pf                                       Input Offset <1 mV     <1 mV      <1 mV                                       Input Bias Current                                                                         500 na    15 μa   <5 μa                                    Open Loop Gain                                                                             >160 dB   150 dB     180 dB                                      Unity Gain Bandwidth                                                                       4 MHz     30 MHz     45 MHz                                      Acquisition Time                                                                           500 nsec  100 nsec   50 nsec                                     Droop Rate   .1 mV/μsec                                                                           1 mV/μsec                                                                             1 mV/μsec                                Slew Rate    90 V/μsec                                                                            130 V/μsec                                                                            180 V/μsec                               Pedestal Error                                                                             2 mV      1 mV       1 mV                                        Hold Mode Settling,                                                                        100 nsec  30 nsec    20 nsec                                     .8%                                                                           Hold Mode Settling,                                                                        200 nsec  60 nsec    35 nsec                                     .015%                                                                         ______________________________________                                    

Analog Switch

Timing controller block 330 controls analog switch 334 which is a set ofanalog CMOS transmission gates. Analog switch 334 must be able to passanalog signals in the -2.5 to +2.5 volt range. With the power rails at-5 volts and +5 volts the analog CMOS transmission gates easily handlethis range. Alternative switch implementations such as controlled CMOSinverters could also be used.

Flash Converter

FIGS. 9-22 schematically show the 7-bit flash converter block 306. Inparticular, FIG. 9 illustrates the overall flash architecture whichincludes an array of 127 comparator cells (labelled 902-1 through902-127), each with a voltage reference input (Vref) connected to a tapon resistor ladder 904 and a signal input V_(in) connected to the signalto be converted (either the output of sample and hold 304 or the outputof error amplifier 312). Adjacent comparator cells 902 are functionallyinterconnected so that only the cell which senses a Vref closest to theinput signal V_(in) will output a logic high to array 906. Encoder 906generates a 7-bit binary output (at ECL type levels) which correspondsto the Vref closest to V_(in). Level translators 908-1 through 908-7translate this to CMOS levels and feed MSB Latch cells 308-1 to 308-7and LSB Latch cells 314-1 to 314-7. Latches 910-1 through 910-7 are fortesting.

The 128 resistors (labelled 904-1 through 904-128) of ladder 904 eachhave a nominal resistance of 3.8 ohms. The total resistance of ladder904 is 486 ohms. With a 5-volt drop the ladder will draw about 10 mA anddissipate 50 mW. The resistors 904 are fabricated from polysilicon witha width of at least 40 um in order to avoid electromigration problems atthe contacts. Voltage references (Vref=+2.5 V and Vref=-2.5 V) driveladder 904 so that the drop across each resistor equals 39.0625 mV,corresponding to a least significant bit (LSB) output. To insure that1000 000 will be the outcome of an input within 19.5 mV (1/2 LSB) of 0volts, resistor 904-65 is center tapped to analog ground (e.g., byreplacing resistor 904-65 with two pairs of parallel connected 3.8 ohmresistors connected in series and tapping the series connection). Tocompensate for this center tap of resistor 904-65, resistor 904-1 isreplaced by a 1.9 ohm resistor (two 3.8 ohm resistors in parallel) andresistor 904-128 is replaced by a 5.7 ohm resistor (3.8 ohm and 1.9 ohmresistors in series). Thus, disregarding any comparator cell input biascurrent, the Vref input to comparator cell 902-1 is -2.480 volts(-2.5+1/2 LSB); the Vref input to comparator cell 902-2 is 1 LSB higherthan to cell 902-1; and so forth up to a Vref input to comparator cell902-64 of -1/2 LSB, a Vref input to cell 902-65 of +1/2 LSB, andcontinuing up to a Vref of 2.441 volts (2.5-3/2 LSB) for cell 902-127.

The output of comparator cells 902 is encoded by encoder 906 which feedsseven level translators and latches 908-1 through 908-7. Only a singleone of comparator cells 902 has a high output due to a segment detectingoutput NOR gate with inputs also from the two adjacent comparator cells;and encoder 906 is just a simple array of NPN transistors with basestied to the comparator cell outputs and emitters tied to the sevenbitlines feeding the level translators/latches 908. Thus when comparatorcell 904-j has the high output, all of the NPN transistors in the jthrow turn on and pull the connected bitlines up about 0.54 volts (from4.46 volts to 5.0 volts) and thereby encode the output. Leveltranslators 908 and latches 308 on the bitlines amplify and translatethe 0.54 volt swings on the bitlines into full CMOS levels and latchthem. The encoding expresses positive numbers with a leading bit equalto 1 and negative numbers in two's complement form with a leading bitequal to 0.

FIGS. 10a-b are a schematic circuit diagram for a comparator cell 902which has first gain stage 1010, second gain stage 1020, latch 1030, andoutput NOR gate 1050. First gain stage 1010 includes NPNemitter-followers 1001 and 1002 for buffering the Vref and Vin inputsignals, respectively, to NPN differential pair 1003-1004, which haveNMOS 1017 as their current source. NMOS 1011 and 1012 provide currentsources, load resistors 1013 and 1014 are made of NiCr, and NPN 1019 isdiode connected. The devices operate with +5 volt (Vcc) and -5 volt(Vee) power supplies.

The outputs of first gain stage 1010 are limited to a swing of about 2.0volts. These feed the inputs of second gain stage 1020 which includesinput NPN differential pair 1021-1022, NiCr load resistors 1023 and1024, NPN switch 1027, resistor 1028, and NMOS current source 1029.Second stage 1020 operates with +5 volts and ground power supplies. Theoutputs of second stage 1020 drive latch 1030, formed with cross-coupledNPNs 1033-1034. NPN 1031 provides the coupling from the collector of NPN1033 to the base of NPN 1034. NPN 1032 couples the collector of NPN 1034to the base of NPN 1033. NMOS 1035 and 1036 are current sources for NPN1031 and 1032, respectively. NPNs 1037 and 1038 provide diodes, NPN 1041is a switch, and resistor 1043 connects NPN 1041 to current source 1029.The latch devices also operate with +5 volts and ground power supplies.

Second stage 1020 and latch 1030 operate as follows. The flash clock(the flash clock is the sum of FLASH1 and FLASH2) is translated to Vbelevels (see FIGS. 14a-b and CLK in FIG. 10b) and drives the base ofswitch NPN 1041. The complement of the flash clock drives the base ofswitch NPN 1027. Thus, prior to a conversion, switch NPN 1027 is on anddifferential pair 1021-1022 is active but switch 1041 is off andcross-coupled pair 1033-1034 are inactive. However, NPNs 1031 and 1032are both active and the result of the comparison of Vref with Vin (whichmay be varying) passes to NOR gate 1050 (to the base of NPN 1051) and tothe NOR gates of the adjacent comparator cells. Once flash clock goeshigh, switch 1027 cuts off the current to differential pair 1021-1022and turns on switch 1041. This activates cross coupled NPN 1033-1034 tolatch in the most current result of the comparison. Note that theswitching and latching involves only current switching in NPN devices,so the voltage swings stay down in the range of 0.5 volt and do notcreate as much noise as comparable CMOS logic switching.

Latch 1030 has three outputs: inverting nodes 1045 and 1046 andnoninverting node 1047. Node 1045 is one of the three inputs for NORgate 1050; inverting node 1046 is an input to the NOR gate of theadjacent comparator cell receiving a higher Vref; and noninverting node1047 is an input to the NOR gate of the adjacent comparator cellreceiving a lower Vref. NOR gate 1050 includes parallel pulldown NPNs1051, 1052, and 1053, plus NMOS current source 1055, logic referencevoltage input NPN 1057, and pullup resistor 1058. The output of NOR 1050connects to a row of encoder 906. The input (base) of NPN 1051 connectsto an inverting output (node 1045) of latch 1030, the input of NPN 1052connects to an inverting output of the latch of the adjacent comparatorcell with a lower Vref, and the input of NPN 1053 connects to thenoninverting output of the adjacent comparator cell with a higher Vref.Hence, the output of NOR gate 1050 is logic low unless all three of NPNs1051-1053 are turned off, and this provides a logical segment detectionin comparators 902 as follows.

NOR gate 1050 in comparator cell 902-j is high precisely when its node1045 is low and node 1045 from cell 902-(j-1) is also low and node 1045from cell 902-(j+1) is high. This corresponds to V_(in) being greaterthan Vref for cell 902-j (and Vref for cell 902-(j-1) which is lower)and being less than Vref for cell 902-(j+1). And in this case NOR gate1050 of cell 902-j being high pulls the jth row of encoder 906 highwhich in turn pulls the appropriate coding columns high. The NOR gatesin all other cells 902-k have at least one of NPNs 1051-1053 turned onto pull the kth row of encoder 906 low and thereby not affect any of thecoding columns. The NOR gates 1050 also provide some error correction.The NOR gate outputs will only be high if Vin is greater than Vref forcells 902-(i-1) and 902-i and if Vin is less than Vref for cell902-(i+1). This requirement on the states of three adjacent cells avoidshaving two adjacent cells output a logic high signal at the same time.Otherwise, if two adjacent cells have high outputs the resulting binarycode could have a value of up to twice the correct value; the threeinput NOR gate prevents this from happening.

FIG. 11 shows the circuitry for level translators 908-1 through 908-7.The corresponding column of encoding array 906 connects to diode 1102into the base of NPN 1105 of differential pair 1105-1106. The base ofNPN 1106 connects to a bias with level midway between the extremes ofthe swing at the base of NPN 1105. The currents through NPNs 1105-1106are mirrored by PMOS mirrors 1110-1111 and 1112-1113 and then NMOSmirror 1114-1115 to drive a CMOS output inverter 1120. FIG. 12illustrates the bias circuit for NPN 1106.

FIG. 13 shows bias generator 1300 for setting gate voltages in thecomparator cells 902. FIGS. 14a-b show the clock generator fortranslating the CMOS level flash clock signal to +1/2 Vbe and -1/2 Vbelevel signals for driving switch NPNs 1027 and 1041 in comparator cells902.

Each of the 127 comparator cells 902 has seven current source NMOSdevices (1012, 1017, 1011, 1029, 1035, 1036, and 1055 in FIG. 10). Thusa large number of equal parallel current sources must be provided toinsure uniform behavior of the comparator cells. FIG. 15 shows astandard base current compensated NPN current mirror 1500 with twooutputs; the resistor current typically is an order of magnitude largerthan the base currents. This current mirror overcomes base current errorsensitivity of a basic NPN current mirror, but has the drawback ofhaving to provide a base current for every output NPN, which becomesintolerable for the 128×7 outputs required by the comparators 902. FIG.16 illustrates a basic NMOS current mirror 1600 which has the advantagesof high packing density and zero bias current, and low drain to sourceoperating voltages when a large number of outputs are required. However,the NMOS current mirror is sensitive to kickback noise. That is, atransient voltage spike at one of the outputs capacitively couples(i.e., a gate-to-drain parasitic capacitor) to gate bias line 1602. Thiscauses a gate bias fluctuation and a current fluctuation in all of theother outputs. The magnitude of the gate bias fluctuation depends uponZ/(Z+Z_(cap)) where Z_(cap) is the impedance of the gate-to-draincapacitor and Z is the impedance to ac ground of gate bias line 1602. Ineffect, a high pass filter exists between each output and gate bias line1602 because Z_(cap) varies as the reciprocal of frequency. Theimpedance Z is the reciprocal of the transconductance of NMOS 1604 ifthe impedance of reference current source 1610 and the output impedanceof NMOS 1604 are large and neglected. Hence, the small transconductanceof NMOS 1604 generally leads to the kickback noise sensitivity of thebasic NMOS current mirror 1600.

The preferred embodiment current mirror 1700, shown schematically inFIG. 17, inserts an NPN current mirror 1705 between reference currentsource 1710 and NMOS 1704 of an NMOS current mirror 1709. This lowersthe impedance to ac ground of gate bias line 1702 because the hightransconductance of NPN 1706 provides a path to ac ground parallelingNMOS 1704. An order of magnitude drop in the impedance may be easilyachieved without a large increase in substrate area occupied by thedevices. Thus current mirror 1700 can provide 20 dB further kickbacknoise rejection plus maintain the advantages of NMOS current mirrors.

The current mirror 1700 operates as follows. NMOS 1714 is matched withNMOS 1704 to provide the same voltage drop for equal currents. NPN 1716and NPN 1726 match NPN 1706, so they form a base current compensatedcurrent mirror with matching NPN 1728 the shunt resistor. NMOS 1724matches NMOS 1704 and 1714 to provide the same voltage drop. Thus theemitter current from NPN 1706 mirrors the reference current from source1710 within a factor that can be taken as 1 presuming a large gain byNPN 1726. Output NMOS transistors 1751, 1752, 1753, etc. match NMOS 1704and have the same gate bias, so the outputs mirror the referencecurrent. Of course, the load devices 1724 and 1728 could be replaced byresistors, but this typically occupies more substrate area.

Current mirror 1700 can be modified in various ways to adapt theseprinciples of kickback noise rejection to other MOS current mirrorcircuits. For example, FIG. 18 shows a basic stacked NMOS current mirroras would be used for high output impedance applications with referencecurrent source 1810 through NMOS 1804-1805 being mirrored by the outputNMOS stacks. FIG. 19 shows a preferred embodiment version 1900 of astacked NMOS current mirror where NPN 1906 provides hightransconductance to lessen kickback coupling. Indeed, simulations on thecurrent mirrors 1800 and 1900 confirm that mirror 1900 provides 31 dB ofadditional kickback rejection.

FIG. 20 illustrates a low current version of current mirror 1700. Thereference current from source 2010 is divided among NMOS devices 2004-1,2004-2, . . . 2004-N so each device 2004-j outputs only 1/N of thereference current.

FIG. 21 shows current mirror 2100 which modifies current mirror 1700 tocompensate for the Early voltage induced errors of NPN 1706. Currentmirror 2100 includes NPN 2107 with a fixed bias set to match the Vce ofNPN 2106 to the Vce of NPN 2116.

FIG. 22 illustrates a PMOS current mirror 2200 which includes thekickback suppression using NPNs. Current mirror 2200 provides the hightransconductance of NPN 2206 in series with NPN 2220 to create the lowimpedance from gate bias line 2202 to ac ground. The reference currentfrom source 2210 is mirrored into NMOS 2212 and then into NMOS 2214,which has twice the gate width of NMOS 2212. Thus twice the referencecurrent passes through NMOS 2214. And NPN 2206 is biased by PMOS 2211 topass the reference current. Consequently, PMOS 2204 and NPN 2220 alsopass the reference current, and this is mirrored by output PMOS 2231 and2232 through gate bias line 2202. NPN 2220 provides a Vbe voltage dropto match that of NPN 2206, and PMOS 2204 matches PMOS 2211.

Current mirror 1700 could be converted to a PMOS current mirror byreplacing NPN with PNP and NMOS with PMOS. Similarly, the other currentmirrors 1900, 2000, 2100, and 2200 could be transformed by P and N typedevice switches.

MSB Latch

MSB latch 308 is a set of seven standard latches 308-1 through 308-7indicated in FIG. 9, which are clocked to load the outputs oftranslators 908-1 through 908-7 at the falling edge of FLASH1. Thefalling edge also cuts off the current to latches 1030 and reappliescurrent to the differential pairs 1021-1022 in the comparator cells 902.This prepares flash converter 308 for another conversion. The outputs ofMSB latch 308 are labelled A1, A2, . . . A7.

DAC

FIGS. 23-30 illustrate various components of DAC 310. As shown in FIGS.23a-d, the DAC includes core 2302, control amplifier 2304, referencecell 2305, and interface 2310. Sample and hold 304, analog switch 334,and error amplifier 312 are also shown in FIGS. 23c-d. DAC 310 usescurrent scaling with the CMOS bits from MSB Latch 308 translated to ECLlevels within interface 2310 which then drive current switches in core2302. The DAC output current feeds error amplifier 312, as shown in FIG.31. FIGS. 24a-d show core 2302 with cells 2401-2415 controlled by thebits from MSB latch 308. FIG. 26 shows the current switch structure 2600for the cells 2401-2404, and FIG. 25 shows the current switch structure2500 for the cells 2405-2415. Each cell 2500 or 2600 has an input NPNdifferential pair 2501-2502 or 2601-2602 tied to a current source madeof biased NPN 2510 and NiCr resistor 2512 or biased NPN 2610 and NiCrresistor 2612. Resistor 2512 is shown as two resistors in series, andresistor 2612 is shown as four resistors in series. When input 2520receives a logic high signal (-0.7 volt), and complementary input 2521receives a logic low signal (-2.1 volts), NPN 2501 turns on and NPN 2502turns off. This steers the current from output 2530 to current source2510-2512 and leaves output 2531 in a high impedance state. Reversedinputs similarly steer the current from output 2531 and leave output2530 in a high impedance state. Cell 2600 is analogous. Thus theswitching in the core cells only steers a constant current and involvesvoltage swings of 1.4 volts. This provides higher speed and lower noisethan is attainable with CMOS switching.

Cells 2405-2415 all have equal current sources (see FIG. 25 withresistor 2512 at 1 Kohms) and correspond to the higher order bits fromMSB Latch 308. A7 (the highest order bit) drives four cells: 2412-2415;A6 drives two cells: 2406-2407; and A5 drives cell 2405. In each case ifthe bit is a 1, then the cell steers the current from DAC output 2430,and if the bit is a 0, the cell steers the current from DAC output 2431.The four cells 2408-2411 provide a constant current, through currentmirrors 2420, to DAC output 2430. This constant output current justoffsets the current absorbed by cells 2412-2415 when bit equals 1 andcorresponds to the fact that a 0 volt input V_(in) leads to a 1000000from flash converter 306.

Cells 2401-2404 (cells as in FIG. 26) have proportionally smallercurrent sources than those of cells 2405-2415 by the use ofproportionally larger resistors 2612: A4 switches half the currentswitched by A5 because resistor 2612 of cell 2404 is about twice thevalue of the resistor 2512 of cell 2405. Similarly, A3 switches half thecurrent switched by A4, A2 switches half the current switched by A3, andA1 switches half the current switched by A2.

FIGS. 27a-c show interface 2310 which translates the CMOS levels of bitsA1 A2 . . . A7 to bipolar levels with a translation cell for eachcurrent cell in core 2302; and FIG. 28 illustrates the translation cell.Interface 2310 also isolates the analog currents in core 2302 from theCMOS switching noise.

FIG. 29 shows the connection of control amplifier 2304 and referencecell 2305 to the core cells.

FIG. 30 shows override register 2320 which simply applies 1000000 tointerface 2310 when the SWITCH signal is low and passes A1 A2 . . . A7from MSB Latch 308 to interface 2310 when the SWITCH signal is high.This control by the SWITCH signal has the advantages of (1) applying allbits A1 A2 . . . A7 simultaneously to the current switches so that DAC310 settles directly toward its final output current rather than huntingas when currents are switched sequentially; and (2) the 1000000 inputholds the output of DAC 310 to its midrange 0 current, which minimizesthe maximum output current change when switched to pass A1 A2 . . . A7.The falling edge of FLASH1 drives the SWITCH signal high, so A1 A2 . . .A7 pass to drive the DAC core current switches and begin the settling ofthe DAC output current to V_(rq) /R. The SWITCH signal returns low onthe rising edge of the ACQUIRE signal which follows the FLASH2 signal byabout 30 nsec. SWITCH going low throws analog switch 334 to disconnectthe output of error amplifier 312 from the input of flash converter 306and reconnect sample and hold 304. Thus the output of DAC 310 settlingback to 0 does not create any noise for the second step conversion. Thesettling precedes a first flash conversion in a second sample of V_(in)(t) by enough time to ready DAC 310 for another conversion. DAC 310takes about 35 nsec to settle to 14-bit accuracy. The linearity of DAC310 depends primarily upon (1) the Early voltage magnitude and matchingamong the NPNs used in the current switching cells, (2) the current gainand matching among the same NPNs, and (3) the quality of the NiCr filmused for the resistors in the cells.

Error Amplifier

Error amplifier 312 includes two serially-connected gain amplifiers withthe first amplifier providing a gain of 4 and the second a gain of 8 foran overall gain of 32. FIG. 31 illustrates the connections of the twogain amplifiers 3100 and 3101 with feedback resistor ratios setting thegains. DAC 310 absorbs current Io to subtract V_(rq), the reconstructedquantized version of V_(in), from V_(in) supplied by sample and hold304. That is, sample and hold 304 supplies a current of V_(in) /R to thevirtual ground at the inverting input of amplifier 3100; and DAC 310absorbs the current Io equal V_(rq) /R. Thus the voltage at node 3110 is-4(V_(in) -V_(rq)). R is about 400 Ω.

Gain amplifier 3100 (and gain amplifier 3101) has a two gain-stagefolded cascode design. The output stage includes level shifting and amodest gain. The input stage develops most of the gain in order tomaintain a high bandwidth while minimizing error sources. The inputstage is a precision stage with low input bias currents and quadcross-coupled input NPN devices. Parallel clamping input stage protectsamplifier 3100 during overdrive conditions; such as when V_(in) appearsat the inverting input without any offsetting current from DAC 310.

FIG. 32 shows gain amplifier 3100 in block form, and FIGS. 33a-d show itin schematic circuit form. Amplifier 3100 includes: bipolar differentialinput stage 3210; CMOS differential input/clamp stage 3220; differentialto single ended stage 3230 which combines bipolar and CMOS devices;output stage 3240; and overdrive protection switches 3250. In a moregeneral configuration CMOS differential stage 3220 could have its inputsconnected to the inputs of the bipolar differential input stage tocreate a two channel amplifier with differing input gain stagesselectable by switches 3250.

Normal operation of amplifier 3100 has switch 3252 closed and bipolarstage 3210 fully biased and in complete control over the output; switch3251 is open to completely debias CMOS stage 3220 which then lacks anycontrol over the output. In contrast, clamp operation of amplifier 3100has switch 3252 open to force bipolar stage 3210 to operate at very lowbias current supplied by source 3253 and exert limited control over theoutput. Operation of bipolar stage 3210 at very low current rather thanturning it totally off permits rapid energization when switching fromclamp operation to normal operation. Also clamp operation has switch3251 closed to energize CMOS stage 3220 which takes control of theoutput. Feedback resistors 3261-3262 and the potential applied toresistor 3262 (ground in FIG. 32) determine the clamp operation outputvoltage (0 volts).

CMOS devices are utilized both to provide matched biasing currents andto sense voltages; this avoids base current errors of bipolar bias andsense circuits and avoids corruption of the matched currents. ExploitingCMOS produces excellent input characteristics like low offset voltagetemperature coefficient and low input current and boosts open loop gain.Speed is the most critical requirement of amplifier 3100, and the NPNdevices have a cutoff frequency of at least 3 GHz. High beta NPNs areused to meet the input bias current conditions. Stacked PMOS devices areused to produce high impedances to achieve large open loop gain in thefirst stage.

FIGS. 33a-d schematically show amplifier 3100 with CMOS stage 3220 andswitches 3250 in FIG. 33a. Zener based bias circuit 3310 in FIG. 33b,bipolar input stage 3210 in FIGS. 33b-c, differential to single endedstage 3230 in FIGS. 33c-d, and output stage 3240 in FIG. 33d. Biascircuit 3310 uses Zener diode D660 and forward biased NPN diode Q596 anddiffused resistor R662 to achieve a temperature stable bias for NPNQ592. NPN Q592 provides a reference current through resistors R618,R657, R619, R705, R706 to a current mirror made of NPNs Q149, Q599, andQ600 plus resistors R597 and R609 and an NPN base bias on line 3312 forother current sources in amplifier 3100. PMOS M602-M603 also mirror thecurrent to provide a PMOS bias on line 3311 for other current sources inamplifier 3100.

The bipolar input stage 3210 includes differential input NPN emitterfollowers Q166 and Q168 driving NPN emitter coupled pair Q165-Q169 withNPNs Q211, Q162 and Q161 connecting them to NPN current source Q156 plusresistor R110. Note that the noninverting input (base of NPN Q166)connects to ground through RX (see FIG. 31) and that the inverting input(base of NPN Q168) connects to the output of DAC 310. Each of the inputscan vary between -2.5 volts and +2.5 volts, but during amplification ofthe quantization error the magnitude of the input difference should beless than 40 mV. However, when DAC 310 is held at a 0 current output,the magnitude of the input difference could be up to 2.5 volts, and CMOSdifferential input stage 3220 provides protection during such overdrive,as described below. The power rails Vee and Vcc for input stage 3210 areat -5 volts and +5 volts.

The differential output signals from stage 3210 pass through NPNshielding devices Q163 and Q164 to differential-to-singled-ended stage3230. Stage 3230 has cascoded PMOS M27 and M30-M32 and a pair of voltagefollowers and a current mirror for conversion to a single-ended outputto drive output stage 3240. One voltage follower is for the load currentmirror and the other is to drive output stage 3240. The voltagefollowers are basically made of NMOS M12, NPN Q181, and NPN Q182 for thecurrent mirror and of NMOS M11, NPN Q184, and NPN Q183 to drive outputstage 3240. Output stage 3240 includes NPNs Q191, Q192, and Q193. Anycurrent and voltage mismatches between these two voltage followers willgenerate error currents causing degradation of open loop voltage gain,offset voltage, and offset voltage temperature coefficient. Idealvoltage followers have 0 input current and maintain equivalentcollector-to-base voltage drops for current mirror devices Q176, Q177,Q178, and Q179 while contributing minimal phase shift.

FIG. 34 shows a standard voltage follower based upon MOS devices forvery high input impedance. Such followers have poor Vgs matches whichcause Vcb mismatch between the NPN mirror devices Q12a and Q13a. Thisgenerates error currents and degrades performance. FIG. 35 shows astandard voltage follower formed with bipolar devices to provide a goodvoltage match. Such followers have relatively low input impedance. Themismatch of the base currents produces an error current that degradesperformance. The two separate current sources for each of the followersin FIGS. 34 and 35 also leads to a source of mismatch and performancedegradation.

The voltage followers of amplifier 3100 (FIG. 33c-d), shown in asimplified form in FIG. 36, are called composite voltage followers (CVF)due to the combination of both MOS and bipolar devices. This arrangementbenefits from the high input impedance of the MOS devices (M0 and M9)while the cross-coupled bipolars (Q4, Q6, Q10, and Q11) improve thematch of the followers beyond that obtainable with MOS devices alone.This improvement works for both DC and transient signal conditions.Improvement in match between the MOS M0 and M9 devices is partiallyaccomplished by providing matched currents to the MOS devices. Thesecurrents match under both DC and transient signal conditions. Becausethe MOS devices have ˜0 input current, no error currents are generatedat the differential to single-ended conversion point, labelled HIP inFIG. 36. This results in improvements in open loop gain, offset voltage,and offset voltage temperature coefficient over that obtainable usingonly bipolar transistors. Match of the followers is improved by the useof one current source to bias both transistors, with further improvementdue to the cross-coupled bipolars biasing the NMOS followers. One of thefollowers (A) feeds the signal from the High Impedance Point (HIP) tothe output stage, the second follower (B) is required as a voltage clampin the current mirror (Q12 and Q13). DC bias current for both followersis provided by current source I7. No special restrictions are placed onthe actual implementation of I7. An NPN or NMOS device is sufficient.The implementation in FIG. 33c-d uses an NPN as the negative biascurrent rail for amplifier 3100 NPN current sources.

The CVF of FIG. 36 operates as follows. The current from I7 is dividedby NPN devices Q4 and Q6. Although this application has the currentequally split between these two devices, other applications may findadvantages in another ratio. Device Q4 provides half of the NMOS M9operating current, while Q6 provides half of the NMOS M0 operatingcurrent. The other half of the M9 current comes from NPN Q11, and theother half of the M0 current comes from NPN Q10. This cross-coupling ofbias for the NMOS followers provides an improved operating point matchfor the NMOS followers M0 and M9 and the current mirror devices Q12 andQ13. This leads to better open loop voltage gain, offset voltage, andoffset voltage temperature coefficient performance of amplifier 3100.The bias current division function of the cross-coupled bipolar devices(that results in an improved amplifier) also divides the load current ofboth followers. Half of the OUTPUT load current comes from M9 throughQ4, with the other half from M0 through Q6. The base current of thecurrent mirror devices Q12 and Q13 load both followers in a similar way.Half of this load current comes from M0 through Q10 and half from M9through Q11. This sharing of load currents between the followers insuresthat the Composite Voltage Followers maintain identical operating pointsleading to better match and an improved amplifier. The PMOS currentmirror (M16-M19) and NMOS current mirror (M14-M15) close the loop aroundthe Composite Voltage Followers. The drain current of M0 is exactlyduplicated as the drain current of M15 (applies to DC and Transientcurrent). The drain current of M0(M15) is composed of:

    I7/2+Iout/2+(IbQ12+IbQ13)/2+IdM15/2=IdM0

The drain current IdM9 of M9 is composed of:

    I7/2+Iout/2+(IbQ12+IbQ13)/2+IdM15/2=IdM9

The end result is (as desired): IdM0=IdM9

The operating currents of the NMOS followers match perfectly due to thecross-coupling of the NPNs (Q4,Q6,Q10,Q11) and the mirroring of M0'sdrain current to M15's drain. The dividing action of the cross-coupleddevices along with the mirroring of M0's drain current insure that M0and M9 see the same load. This applies to both DC and transientconditions.

Both NMOS followers see the same transients. This improves the settlingtime because the CVF presents a symmetric load to the mirror devices Q12and Q13. Any asymmetry would cause undesirable ringing in the settlingwaveform. Any transient voltage or current at the HIP would be mirroredover to the other input device, but symmetry will lead to less ringing.

FIG. 37a shows a PNP version of the Composite Voltage Follower and FIGS.37b-c show all NPN and all NMOS versions. In particular, the all NPNversion of FIG. 37b has the same cross coupling and consequent symmetrybut will not have the high input impedance of the CVF of FIGS. 36 and37a. The all NMOS version of FIG. 37c will not have the high speed ofthe CVF of FIGS. 36 and 37a. Also, in all of the CVFs the ratio ofcurrent division by the cross coupling could be changed by ratioing theemitter areas or gate widths of the cross-coupling devices.

As shown in FIGS. 33, the output of the Composite Voltage Followerdrives the bases of NPNs Q191 and Q193 in output stage 3240 in the lowerrighthand portion of FIG. 33d. The output terminal Out of output stage3240 feeds back to CMOS stage 3220. Resistor 3261 of FIG. 32 correspondsto R167 in FIG. 33b, and resistor 3262 of FIG. 32 corresponds to theseries resistors R607, R693, R694, R695, and R696. CMOS stage 3220 hasas inputs differential NMOS pair M621 and M639 tied to NPN currentsource Q627 and Q626 plus resistor R630 of FIG. 33a. The differentialoutputs of the NMOS pair connect to the differential outputs of bipolarstage 3210 at cascode PMOS M27, M30, M31, and M32.

The clamp terminal in FIG. 32 corresponds to the Clamp terminal at thelefthand edge of FIG. 33a. Switches 3251 and 3252 of FIG. 32 areimplemented primarily by NPN Q625 driven by differential PMOS pairM645-M646 with current mirror load of NMOS M641-M642 in FIG. 33a. Inparticular, a low (ground) signal at terminal Clamp turns on M645, turnsoff M646, and pulls node 3303 up to about -2.2 volts (at roomtemperature) because the diode stack NPNs Q631, Q632, Q619, and Q638limits anything higher. This turns on NPN Q625 and thus steers thecurrent supplied by PMOS current source M614 away from PMOS M620 andinto NPN current source Q626. With no current supplied by M620, all ofthe current for NPN source Q156 (FIG. 33c) comes from the bipolardifferential pairs and puts bipolar stage 3210 into maximum gaincondition. Also, Q625 supplying the current to source Q626 implies Q627turns off and inactivates CMOS differential pair M621-M639, so CMOSstage 3250 presents high impedance outputs.

Conversely, a high (+5 volts) signal at terminal Clamp turns on M646,turns off M645, and pulls node 3303 down to about -3.6 volts (becausethe base of NPN Q633 is at about -2.9 volts) which turns off NPN Q625and thus steers the current supplied by PMOS M614 into PMOS M620 andthen into NPN current source Q156. Supplying this current to Q156 leavesonly a small trickle current to be drawn from the bipolar differentialpairs, and bipolar stage 3210 remains active but with very small gain.With Q625 turned off, NPN Q627 supplies the current from source Q626 toNMOS pair M621, M639. The output of the NMOS pair will overpower that ofthe reduced gain bipolar stage 3210, and the resistor feedback from Outto the NMOS pair will hold amplifier 3100 at a 0 volt output. CMOS stage3220 has lower transconductance than bipolar stage 3210, so theamplifier is more stable in the clamp mode.

In summary, DAC 310 and amplifier 3100 operate together as follows.Initially, a low SWITCH signal holds the input to DAC 310 at 1000 000 tothus its output at 0 current, and a low Clamp signal puts amplifier 3100in clamp mode with CMOS stage 3220 holding the output at 0 volts despiteany nonzero V_(in) input from sample and hold 304. When the SWITCHsignal goes high the encoded quantized version of V_(in) (A7A6 . . . A1)enters DAC 310 and the output current of DAC 310 begins settling toV_(rq) /R where V_(rq) equals the reconstructed quantized version ofV_(in). At this time the inputs to bipolar stage 3210 of amplifier 3100are ground at the noninverting input and DAC 310 output current plusV_(in) /R current from sample and hold 304 at the inverting input.Amplifier 3100 remains in clamp mode for a delay period of about 10nsec. This permits other switching noise to attenuate and the DAC 310output current to get close to -V_(rq) /R to avoid overdrive saturationof bipolar stage 3210. Then the Clamp signal goes high to disable CMOSstage 3220 and jump the gain of bipolar stage 3210. Bipolar stage 3210then settles to its amplification of the settling quantization error.DAC 310 settles to 14-bit accuracy (0.3 mV) within about 50 nsec. Thebipolar stage 3210 has a high cutoff frequency and amplifier 3100 tracksthe settling quantization error. Similarly, amplifier 3101 tracks theoutput of amplifier 3100 so that the overall output of error amplifier312 settles to within 4 mV of final output within 80 nsec.

Amplifier 3100 could be configured for general purpose use. The inputsto the bipolar and NMOS differential pairs could be tied together as thedifferential inputs, and the digital signal at terminal Clamp just aselection between the bipolar and NMOS inputs. Thus amplifier 3100 is achannel selectable amplifier with the two channels providing differentperformance. The bipolar channel provides high speed and low noiseoperation, while the CMOS channel provides high input impedance.

LSB Latch

LSB latch 314 is a set of seven standard latches, 314-1 through 314-7indicated in FIG. 9, which are clocked to load the outputs oftranslators 908-1 through 908-7 at the falling edge of the FLASH2 clock.The falling edge also cuts off the current to latches 1030 and reappliescurrent to the differential pairs 1021-1022 in comparator cells 902, andso prepares flash converter 308 for another conversion. The outputs arecalled C1, C2, . . . C7.

Subtractor

Subtractor 316 is simply a binary adder that subtracts 0000 010 from A7A6 . . . A1 by adding the two's complement of 0000 010, namely 1111 110,to A7 A6 . . . A1 and calling the result B12 B11 . . . B6. The carry bitis called CR1:

    ______________________________________                                             A7        A6     A5      A4  A3     A2  A1                               +    1         1      1       1   1      1   0                                CR1  B12       B11    B10     B9  B8     B7  B6                               ______________________________________                                    

Subtracting 0000 010 compensates for the 1000 000 output of flashconverter 306 with a 0 volt input during the second flash conversion tocreate the least significant bits. A more detailed explanation appearsin the discription of error correction block 318. Subtractor 316performs the substraction within about 6 nsec, and during this time DAC310 has begun to settle to its V_(rq) /R output current, but erroramplifier 312 remains clamped.

Error correction

FIG. 38 schematically shows the circuitry of error correction block 318.This logic implements part of the following procedure and many otherimplementations also exist and can be automatically generated by logicdesign programs. A7 A6 A5 . . . A1 denotes the output of flash converter306 on the first conversion of input V_(in) and held in MSB latch 308;that is, A7 A6 . . . A1 is the binary coding of the quantized versionV_(q) of V_(in) with quantization levels spaced 39.0625 mV and withV_(in) equal to 0 volts ideally yielding A7 A6 . . . A1 equal to 1000000 due to the bipolar input range. DAC 310 reconstructs the quantizedversion V_(q) of V_(in) from the binary code; call this V_(rq). Thus,ideally, V_(in) and V_(rq) only differ by at most 19.53125 mV (one halfof a 39.0625 mV quantization level). Error amplifier 312 outputs32(V_(in) -V_(rq)) and this ideally falls in the range of -0.625 V to+0.625 V and leaves room for error as will be described below. FIG. 39heuristically illustrates how V_(in) within a quantization level willlead to 32(V_(in) -V_(rq)) within the -0.625 to +0.625 volt range forthe second conversion. Flash converter 306 converts 32(V_(in) -V_(rq))to C7 C6 . . . C1 which LSB latch 314 stores. Due to the amplificationby 32, the quantization level separation of 39.0625 mV on the secondconversion corresponds to a 1.22 mV level in V_(in) -V_(rq). Again, ifV_(in) -V_(rq) is 0, then C7 C6 . . . C1 equals 100 0000.

Subtractor 216 subtracts 0000 010 from A7 A6 . . . A1 and the result istermed B12 B11 . . . B6 with the carry termed CR1; the carry termresults from the subtraction being performed by addition of the two'scomplement of 0000 010, namely 1111 110. Thus V_(in) equal to 0 wouldideally have B12 B11 . . . B6 equal to 0111 110 and CR1 equal to 1. Thissubtraction of 0000 010 compensates for C7 C6 being 10 when thequantization error V_(in) -V_(rq) equals 0. Error correction block 318(FIG. 38) adds C7 C6 to B12 B11 . . . B7 B6 to yield D12 D11 . . . D7 D6and with carry called CR2:

    ______________________________________                                             B12       B11    B10     B9  B8     B7  B6                               +                                        C7  C6                               CR2  D12       D11    D10     D9  D8     D7  D6                               ______________________________________                                    

Lastly, the final output by output buffer 320 will be D12 D11 . . . D1where D5=C5, D4=C4, D3=C3, D2=C2, and D1=C1. Also, the exclusive OR ofCR1 and CR2 outputs as OR (overrange).

To clarify the foregoing, consider an example in the ideal case oferrorless devices. Let V_(in) be +1.1000 volts. First, 1.074 volts isthe highest quantization level which does not exceed +1.1000 volts; thusflash converter 306 will output 1011 100 because 11 100 is binary for 28and 28 times 39.0625 mV equals 1.09375 volts which is the midpointbetween the quantization levels for codes 28 and 29. The leading 1 inthe 1011 100 output just represents the fact that V_(in) is positive;recall that a 0 input generates a 1000 000 output and negative inputsgenerate leading 0 outputs. So A7 A6 . . . A1 equals 1011 100. If thiswere expressed in terms of a quantization with quantization levelsseparated by 1.22 mV (as in the final output of converter 300), then thecode would simply be 1011 1000 0000 because 11 1000 0000 is binary for28 times 32 and 28 times 32 times 1.22 mV equals 1.09375 volts.

Subtractor 316 adds 1111 110 and 1011 100 to give B12 B11 . . . B6 equalto 1011 010 with a carry to make CR1 equal to 1. Note that CR1 is always1 unless A7 A6 . . . A1 were 0000 000 or 0000 001 which corresponds toV_(in) being about -2.5 volts or out of range and below -2.5 volts.

Next, DAC 310 takes the 1011 100 input and reconstructs +1.09375 volts,the first quantized version of V_(in) and previously called V_(rq). Thenerror amplifier 312 amplifies the quantization error (V_(in) -V_(rq)) of+0.00625 volt by 32 to yield +0.2000 volt. Now +0.176 volt is thehighest quantized level below +0.2000 volt, so flash converter 306 willconvert 0.2000 to an output of 100 0101 because 101 is binary for 5 and5 times 39.0625 mV equals 0.1953 volt which is the midpoint between thequantization levels for codes 5 and 6. Again, the leading 1 representsthe fact that the input was positive. C7 C6 . . . C1 equals 100 0101.Because +0.2000 volt is 32 times +0.00625 volt and 39.0625 mV is 32times 1.22 mV, the first quantization error (V_(in) -V_(rq)) itselfquantizes as 00 0101 in terms of 1.22 mV separated quantization levels.Thus the 00 0101 directly added to the 1011 1000 0000 from the 1.22 mVlevel version of the first quantization gives the final output of 10111000 0101. Thus the leading 1 for a second flash conversion output mustbe compensated if C7 C6 . . . C1 is to be added to yield the finaloutput. The subtraction of 01 from A7 A6 . . . A1 to form B12 B11 . . .B6 is just this compensation; furthermore the increment of the index by5 expresses the first quantization in terms of 1.22 mV levels. Note thatthe maximum input to flash converter 306 on the second flash conversionis 625 mV, so the maximum output is 101 0000 with the leading 1 againindicating a positive input. This means that the most significant twobits C7 and C6 do not (with errorless devices) contain any informationbeyond the already-compensated sign of the first quantization error andcan overlap B7 and B6. Hence, D12 D11 . . . D1 as the sum of B12 B11 . .. B6 and C7 C6 . . . C1 will be the correct result previously noted:

    ______________________________________                                                1011          010                                                     +                     100     0101                                                    1011          1000    0101                                            ______________________________________                                    

and the carry CR2 equals 0. CR2 will always be 0 unless B12 B11 . . . B6is 1111 111 or 1111 110 which means A7 A6 . . . A1 must have been 0000000 or 0000 001, again V_(in) was about -2.5 volts. As previously noted,CR1 is always 1, so the exclusive OR of CR1 and CR2 is 0.

The CR1 and CR2 bits provide out of range detection of V_(in) asfollows. If V_(in) exceeds +2.5 volts, then the first flash conversionyields A7 A6 . . . A1 equal to 1111 111 and the quantization error isgreater than +39.0625 mV because DAC 310 reconstructs 1111 111 as2,4609375 volts, the highest quantization version. Hence, erroramplifier 312 outputs a voltage exceeding +1.25 volts, and the secondflash conversion output C7 C6 . . . C1 is at least 110 0000. Subtractor316 computes B12 B11 . . . B6 as:

    ______________________________________                                                       1111   111                                                     +              1111   110                                                     1              1111   101                                                     ______________________________________                                    

So CR1 equals 1. Adding B12 B11 . . . B6 and C7 C6 to generate D12 D11 .. . D6:

    ______________________________________                                                       1111   101                                                     +                      11                                                     1              0000   000                                                     ______________________________________                                    

And CR2 also equals 1. Thus the exclusive OR of CR1 and CR2 is 1 whichindicates overflow/underflow, and D12 D11 . . . are 0's so it is anoverflow.

Similarly for V_(in) less than -2.5 volts: A7 A6 . . . A1 is 0000 000and error amplifier 312 outputs a voltage less than -1.25 volts. Thesecond flash conversion outputs at most 011 1111. Subtractor 316computes B12 B11 . . . B6 as:

    ______________________________________                                                       0000   000                                                     +              1111   110                                                                    1111   110                                                     ______________________________________                                    

and CR1 is 0. The computation of D12 D11 . . . D6:

    ______________________________________                                                       1111   110                                                     +                      01                                                                    1111   111                                                     ______________________________________                                    

and CR2 also is 0. Hence, the exclusive OR of CR1 and CR2 again is 1 andindicates the overflow/underflow, and D12 D11 . . . are 1's so it is anunderflow.

Nonideal devices in converter 300 may lead to errors in the output, butthe foregoing procedure can correct for the most common ones. Inparticular, the most common source of error lies in the accuracy offlash converter 306, and the headroom (see FIG. 39) available in flashconverter 306 on the second flash conversion permits the correction asfollows. If flash converter 306 outputs a code that is 1 LSB higher thanit should be, then DAC 310 will reconstruct V_(rq) that is 39.0625 mVhigher than the true first quantization of V_(in), and error amplifier312 will output an amplified quantization error that is 1.25 volts lowerthan it should be. Thus the second quantization by flash converter 306is one lower in C6 than it should be, and this precisely cancels theoriginal code error of 1 LSB too high. An example will clarify:

Let V_(in) be +1.1000 volts as in the previous example, then the truefirst quantization level is 1.074 volts and flash converter 306 shouldoutput 1011 100. But presume flash converter 306 fails to be trulylinear and outputs 1011 101 for this input. Then DAC 310 willreconstruct V_(rq) using the erroneous code 1011 101 and output 1.13281volts as V_(rq). Now the quantization error V_(in) -V_(rq) equals-0.03281 volts rather than the +0.00625 volts that would follow from acorrect code. Error amplifier then amplifies this quantization error to-1.05 volts rather than the +0.200 volts following from a correct code.Note that this falls out of the expected errorless range of -0.625 to+0.625 volt. Now flash converter 306 quantizes -1.05 volts as -1.0547volts which is -27 times 39.0625 mV and outputs 0100101 because 100101is the two's complement of 011011 which is binary for 27 and the leading0 indicates a negative number. (Of course, the nonlinearity of flashconverter 306 may again affect the least significant bit.) Then thecomputations are as follows. Substractor 316 finds B12 B11 . . . B6:

    ______________________________________                                                       1011   101                                                     +              1111   110                                                     1              1011   011                                                     ______________________________________                                    

This compares to B12 B11 . . . B6 equal to 1011 010 for the correct codecase. Next, error correction 318 adds C7 C6 to find D12 D11 . . . D6:

    ______________________________________                                                       1011   011                                                     +                      01                                                                    1011   100                                                     ______________________________________                                    

and filling in the C5 . . . C1 yields the final outputs as 1011 10000101 which is the correct final output.

A similar correction takes place if flash converter 306 outputs a code 1LSB too small. Again using the example of V_(in) equal to +1.1000 volts:The first flash converter 306 output would incorrectly be 1011 011 andDAC 310 would reconstruct V_(rq) as 1.0547 volts. Then the firstquantization error (V_(in) -V_(rq)) would be +0.0453, and erroramplifier 312 would output +1.45 volts for the second flash conversion.The second flash converter 306 output would be 110 0101 because 100101is binary for 37 and the leading 1 indicates a positive. The computationof B12 B11 . . . B6 is:

    ______________________________________                                                       1011   011                                                     +              1111   110                                                     1              1011   001                                                     ______________________________________                                    

and the addition of C7 C6 (11) to yield D12 D11 . . . D6:

    ______________________________________                                                       1011   001                                                     +                      11                                                                    1011   100                                                     ______________________________________                                    

Filling in C5 . . . C1 gives a final output of 1011 1000 0101 whichmatches the correct output; of course, the least significant bit couldbe different due to the nonlinearity of flash converter 306 on thesecond flash conversion.

The maximum correctible error from incorrect code on the first flashconversion is 1.5 LSB because the headroom on the second flashconversion (see FIG. 39) is 1.875 volts, both for positive and negative,and this equals 32 times 1.5 times 39.0625 mV.

Overflow/Underflow

Overflow/underflow block 324 is just an exclusive OR of CR1 and CR2 asexplained in the description of error correction block 318. FIG. 40shows thirteen flip-flops for storing the seven bits D12 D11 . . . D6from error correction block 318, the five least significant bits C5 C4 .. . C1 (D5 D4 . . . D1) from LSB latch 314, and the exclusive OR of thetwo carries CR1 and CR2 (upper righthand corner). The LOADOP signalclocks the flip-flops.

Output Buffer

Output buffer 320 includes fourteen drivers each the same as driver 4100shown in FIG. 41. One driver for each of outputs D12 D11 . . . D1, onefor the exclusive OR output of overflow/underflow block 324, and one forthe IRQ (interrupt request) signal. Driver 4100 operates with CMOSdigital power levels: between +5 volts and ground. The data bits D12 D11. . . D1 and exclusive OR output enter driver 4100 at the IN terminaland an enable signal at the EN terminal controls driver 4100. Driver4100 provides NPN output transistors 4102-4103 to drive capacitive loadsbeyond the capabilities of simple CMOS drivers plus also provides alower output voltage V_(OL) than prior art BiCMOS driver 4200 shown inFIG. 42. In particular, the simple driver of 4200 cannot achieve lowV_(OL) levels, such as less than 0.4 volts, especially at lowtemperatures, due to the base emitter drop of NPN 4203 when it is on andpulling the output low. FIG. 43 shows driver 4300 which is a version ofdriver 4100 simplified by the removal of the enable circuitry (NMOS4150-4156) and the ESD protection NPNs 4104-4105.

Prior art driver 4200 operates as follows: a high input at IN invertsthrough CMOS inverter 4206-4207 to a low and thus turns on PMOS 4210 andturns off the NMOS 4211-4212. PMOS 4210 on drives the base of NPN 4202high to turn on NPN 4202 and pull up output terminal OUT. The PMOS 4210on also pulls up the gate of NMOS 4213 which turns on NMOS 4213 to pullthe base of NPN 4203 to ground and keep NPN 4203 off. NMOS 4211-4212 offisolate output terminal OUT from the bases of NPNs 4202-4203; and OUT ishigh. Conversely, a low at input IN inverts through CMOS inverter4206-4207 to a high that turns off PMOS 4210 and turns on NMOS4211-4212. NMOS 4212 on connects the base and collector of NPN 4203together to form a diode and pull OUT down to about 0.7 volts but nolower: this is the V_(OL) problem. During switching, the base chargemust be rapidly removed to avoid delays, and NMOS 4211-4213 accomplishthis.

Drivers 4100 and 4300 include the same devices as driver 4200 but haveadditional circuitry to generate a low output V_(OL) lower than 0.7volts by enhanced driving of the base of output NPN 4303. In particular,NMOS 4321 and resistor 4320, in addition to the diode connection throughNMOS 4312 (plus diode 4322), drive the base of NPN 4303. Drivers 4100and 4300 operate as follows. A high input at IN will invert and turn onPMOS 4310 to drive the base of NPN 4302 and will turn off NMOS 4311-4312and also NMOS 4321; this operates in the same manner as driver for ahigh input. Conversely, a low input at IN inverts to turn on NMOS 4312which makes the connection of base and collector of NPN 4303 (throughdiode 4322) to form a diode and pull OUT down to about 1.4 volts,analogous to the operation of driver 4200. But the low at IN alsoinverts to turn on NMOS 4321 which supplies drive from Vdd throughresistor 4320 to put NPN 4303 into saturation and thereby drop thecollector-to-emitter voltage to about 0.1 volt. This saturation pullsOUT to about 0.1 volt. Diode 4322 prevents the drive by NMOS 4321 fromshunting directly to OUT and lessening its effect. Lastly, smallresistors may be inserted between OUT and each of the NPNs 4102-4103 and4302-4303 in order to reduce inductive (from bond wires) kickback undercapacitive loads.

Driver 4100 operates in the same manner as driver 4300 when the EN inputis high due to NMOS 4154-4156 and PMOS 4152 all being turned on and NMOS4153 being turned off. Conversely, EN low turns off PMOS 4152 to isolatePMOS 4110 and keep NPN 4102 off, turns on NMOS 4153 to keep NPN 4103off, and turns off NMOS 4154-4156 to isolate OUT and stop base driverNMOS 4121. That is, driver 4100 presents a high impedance at OUT.

The fourteen drivers 4100 within output buffer 320 are arranged alongthe outer edge of the silicon die containing the circuitry of converter300. The enable signal for the drivers propagates along the die edge sothat the drivers turn on sequentially with a small (<1 nsec) delaybetween turn ons to lessen ground bounce and other noise thataccompanies the power switching. FIG. 44 shows a layout of converter 300with fourteen drivers marked 4401-4412 for D1 through D12, 4413 for theexclusive OR, and 4414 for IRQ. Signals originate in area 4450 andpropagate in the direction of the arrows.

Lastly, the output format follows from the state of external signal A0:A0 low has buffer 320 output a 12-bit word as described, and A0 highsplits the 12-bit word into two 8-bit words with the second word havingfour trailing 0s. Buffer 320 multiplexes the two 8-bit words.

Timing, Controller, and Oscillator

Timing controller and oscillator block 330 includes timing generator4500 shown in FIG. 45 and made of seven oscillator cells 4501-4507, eachof the structure shown in FIG. 46a as cell 4600. Cell 4600 basicallygenerates a timed delay by sensing when the charge on a capacitor beingcharged by a constant current source reaches a threshold value. Varyingthe capacitance or the current or both varies the time interval. In moredetail: a constant current of 65 uA is mirrored into PMOS 4602(suggested by the broken line PMOS 4601 diode in FIG. 46a); thus whenthe signal at terminal CNTRL switches low, this mirrored current passesthrough turned-on PMOS 4604 and begins charging up a capacitor(suggested by broken line capacitor 4607) at terminal CAP. The voltageat CAP increases linearly with time. Now the NMOS differential pair4611-4612 with PMOS current mirror load 4615-4616 form a comparator withone input, the gate of NMOS 4611, connected to CAP and the other input,the gate of NMOS 4612, connected to a reference voltage of Vdd/2 voltssupplied by a voltage divider to terminal BIAS2.8. Consequently, whenthe voltage at CAP is increasing from 0 towards Vdd/2 volts, thecomparator output at node 4620 remains low and the inverters 4621-4622buffer this to a low at terminal OUT, plus inverter 4623 inverts this toa high at terminal #OUT. Also, inverter 4621 inverts the low at node4620 to a high that keeps PMOS 4630 off. Now when the voltage at CAPapproaches Vdd/2 volts, the comparator begins switching to a high outputat node 4620, and inverter 4621 inverts this to a low which turns onPMOS 4630 to supply a large current to help rapidly charge up capacitor4607. That is, PMOS 4630 provides positive feedback and thereby sharpensthe transition; see FIG. 47 showing the voltage at CAP for variouscapacitors. CNTRL low also keeps NMOS 4605 turned off, but when theCNTRL switches high, NMOS 4605 will turn on to discharge capacitor 4607to ground. Further, a high signal at terminal MR (master reset) willalso discharge capacitor 4607 to ground. In short, when CNTRL is high,CAP is low, OUT is low, and #OUT is high; and when CNTRL is low, CAPramps up, OUT goes high after the ramp delay, and #OUT goes low afterthe ramp delay.

The comparator 4611-4612 plus 4615-4616 detection could be replaced by asimple inverter designed to switch at a particular threshold as shown byinverter 4630 in FIG. 46b. This alternative eliminates two devices andthe bias line from cell 4600; the threshold of inverter 4630 can beadjusted by setting the ratio of the gate widths of the PMOS 4631 andNMOS 4632 making up inverter 4630. The comparator approach of cell 4600permits accurate control of the switching point by control of the biaspoint which can be placed at levels other than Vdd/2. With a comparatorthe bias may be referenced to a fraction of Vdd and thus at highersupply voltages the bias point is higher and the pulse width is almostconstant; in contrast, with an inverter the this is less tightlycontrolled because the threshold has a greater variation with respect tosupply voltage. Further, the dependence of carrier mobility ontemperature implies a general slowing down of devices with increasingtemperature, so providing a current to mirror into PMOS 4602 that varieswith temperature in a desired way will yield a pulse width that variesas desired with temperature. Further, the current mirror could havedifferent size devices for different cells so that the capacitors wouldnot have to be varied in size for the different time intervals required,and the current mirror could be realized with bipolar transistors. Andreplacing the inverter fed by CNTRL with more complex gates can providefor further control of the timing.

FIG. 45 shows the seven oscillator cells 4501-4507 arranged sequentiallywith the #OUT of each cell feeding the CNTRL input of the next cell sothe cells activate in sequence. OR gates 4511-4517 each has inputs ofthe CNTRL and OUT of the corresponding cell; thus an OR gate goes lowprecisely when CNTRL goes low and OUT has not yet switched high due tothe ramp delay. Because the #OUT signal is one gate delay from the OUTsignal, each OR gate will be high before the succeeding OR will go lowand the sequence of pulses from the OR gates will be nonoverlapping.Timing diagram FIG. 5 illustrates the outputs of the OR gates with theirfollowing inverters which have large size for driving large loads: ORgate 4511 provides the 30 nsec low-going HLDSTTL (holdsettle) pulse ofthe second from the bottom panel of FIG. 5, OR gate 4512 plus inverterthe 28 nsec FLASH1 pulse of the third from the bottom panel, OR gate4513 plus inverter the 80 nsec DACSTTL pulse of the fourth from thebottom panel, OR gate 4514 plus inverter the 28 nsec FLASH2 pulse of thefifth from the bottom panel, OR gate 4515 plus inverter the 20 nsecLOADOP pulse in the sixth from the bottom panel, OR gate 4516 plusinverter the 100 nsec ACQUIRE pulse of the seventh from the bottompanel, and OR gate 4517 plus inverter the 20 nsec EOC pulse of the toppanel.

Of course, timing generator 4500 could have been realized by anoscillator driving a ripple counter with decoding the count to providethe timing pulses; however, use of an oscillator (with a 10 nsec period)would have created periodic switching noise which the capacitor chargingof cells 4501-4507 avoids.

FIG. 48 illustrates schematically controller 4800 within block 330.Controller 4800 receives the external control signals of chip select(#CS, active low), output enable (#OE, active low), convert (#CONV,active low), plus internal signals FLASH1, FLASH2, and EOC from timinggenerator 4500, and generates the internal control commands CNTRL("control" which drives timing generator 4500), IRQ ("interrupt request"which drives sample and hold 304 plus an external bus driver),flashclock, and Outputen (enabling output drivers in buffer 320).Controller 4800 operates as follows: first, when #CS is high at terminal4802, then both NOR gates 4804-4805 are low and this holds Outputenterminal 4808 low and feeds low data to flip-flops 4810-4811. The #Qoutput of flip-flop 4810 drives the CNTRL signal, so flip-flop 4810 withlow data implies CNTRL remains high and keeps timing generator 4500 fromstarting a new cycle and converter 300 becomes idle.

Now presume #CS is low. A high signal at #CONV terminal 4812 also drivesNOR gate 4804 low to feed low data to flip-flops 4810-4811 to preventtiming generator 4500 from starting a new cycle.

When #CONV switches low, NOR gate 4804 goes high, and flip-flops4810-4811 have a high at their data inputs. NOR gate 4804 going highalso propagates (presuming EOC at terminal 4822 is low) through theinverter chain made of NAND gate 4830 and inverters 4831-4835 to clockflip-flops 4810-4811 about 8 nsec after the high at their data inputs;this delay insures the data input is high and filters out very short#CONV pulses. Thus a low going #CONV pulse of duration greater than 8nsec makes CNTRL go low and IRQ go high about 10 nsec after #CONVswitches low, and these values are held in flip-flops 4810-4811 untilreset. Note that EOC is low because CNTRL was high and all capacitors inthe oscillator cells are in reset condition giving a low output. Asdescribed previously, CNTRL going low activates timing generator 4500which then outputs the pulses of FIG. 5 to drive a conversion cycle byconverter 300. Also, IRQ going high switches sample and hold 304 intohold mode, so the aperture delay of converter 300 is the delay from CONVto IRQ plus the switching in sample and hold 304. The aperture jitter iskept to a very low level by the sharp thresholds of the inverter chain.Note that the external input terminals #CS, #EN, #CONV, and #A0 each hasa translation buffer for conversion from TTL (0.8 volt low and 2.0 volthigh) to digital CMOS levels, and the typical 8 nsec delay includes thistranslation.

The end-of-conversion pulse EOC from timing generator 4500 feeds backinto controller 4800 at terminal 4822, and if #CONV remains low, thenthe EOC pulse triggers another conversion, but if #CONV has returnedhigh, then EOC has no effect. In particular, with #CONV low, a highgoing edge of EOC will propagate through inverter 4840, NAND gate 4830,and inverter chain 4831-4835 to drop the clock input to flip-flops4810-4811 low. The high going edge of EOC will also switch AND gate 4842high, and thus drive OR gates 4844 and 4848 high to reset flip-flop 4810and switch CNTRL high. CNTRL going high will put a low at the input ofthe AND gate 4842 terminating the EOC pulse with a propogation delay ofgates 4842, 4844, 4848, and 4810. The reset of flip-flops 4810-4811overrides any other signal. Then the falling edge of the EOC pulse willpropagate through the same chain to drive the clock inputs of flip-flops4810-4811 high and clock in the highs (from #CONV low) at their datainputs and thereby drive #CNTRL low to start another cycle by timinggenerator 4500. Thus # CONV held low results in a continuous conversionmode by converter 300.

ACQUIRE pulse going high at terminal 4852 from the timing generator 4500resets flip-flop 4811 to put IRQ low until the next cycle. For theduration of the time that CNTRL remains high the converter will notstart a new cycle because the EOC signal from terminal 4822 blocks NANDgate 4830.

Controller 4800 just ORs FLASH1 and FLASH2 from timing generator 4500and input at terminals 4861-4862 to create FLASHCLK at terminal 4863 todrive flash converter 306. The falling edge of FLASH1 also clocksflip-flop 4871 to load the data held by flip-flop 4810 (#CNTRL) andoutput this through AND (which is for testing purposes only) to 4873 assignal SWITCH. This SWITCH signal releases DAC 310 from the 1000 000input (see FIG. 30) and switches analog switch 334 to direct the outputof error amplifier 312 to flash converter 306 to set up converter 300for the second conversion at FLASH2.

FIG. 49 recapitulates the overall timing for converter 300 forcontinuous conversion operation as represented by CONV remaining low inthe first panel. The falling edge of CONV drives controller 4800 after adelay of 6 nsec through NOR 4804, NAND 4830, and inverters 4831-4835 tosimultaneously clock flip-flops 4810 and 4811 to switch CNTRL low andIRQ high (second and third panels of FIG. 49). IRQ going high turns onan output bus driver to signal an interrupt to the microprocessor orother signal processors being fed conversions by converter 300. IRQswitching high also drives level translator 725 in sample and hold 304which switches transmission gate 721 to disconnect the output ofamplifier 602 and thus begin amplifier 604 holding V_(in) on capacitor606.

CNTRL going low starts a cycle of timing generator 4500 and includesdriving HLDSTTL low after one OR gate 4511 switching delay (fourth panelof FIG. 49). The disconnection by transmission gate 721 in sample andhold 304 contributes a finite charge injection into node 606 and HLDSTTLprovides a settling time of 30 nsec before returning high to start thenext timing pulse. The comparators of Flash converter 306 have been andcontinue tracking the output of sample and hold 304 and sending aquantized version to the NPN encoder of Flash converter 306.

One gate delay after HLDSTTL returns high FLASH1 goes high to driveFlashclk high and have the comparators and NPN encoder latch in the 7bits encoding the quantized output of the comparators. The falling edgeof FLASH1 (28 nsec later) drives down FLASHCLK to latch the 7 bits inthe CMOS latches (MSB Latch 308) but releases the comparators and theencoder array.

One gate delay after FLASH1 returns low DACSTTL switches high to startan 80 nsec settling time pulse; see fifth and sixth panels of FIG. 49.The falling edge of FLASH1 also after a gate delay clocks flip-flop 4871to drive SWITCH high in controller 4800. SWITCH going high performsthree functions: (1) it switches the input of the DAC from 1000000 tothe 7 bits held by MSB Latch 308 and thus DAC begins slewing to itsfinal output, (2) activates Subtractor 316 to subtract 0000010 from the7 bits in MSB Latch 308, and (3) throws analog switch 334 to feed theoutput of error amplifier 312 to flash converter 306 instead of theoutput of sample and hold 304. Thus the flash converter comparators andencoder array are now tracking the output of error amplifier 312 whichis still clamped to 0 volts. The subtraction in Subtractor 316 generatesnoise, but is completed within 6 nsec.

After a delay of 10 nsec from SWITCH going high to allow noise at theinput to error amplifier 312 due to various switching happening on thechip to subside (including subtractor, timing generator, switch, and DACoutput), the clamp is released from error amplifier 312 (see eighthpanel of FIG. 49) which then begins to settle to amplifying thedifference of the DAC output (still settling but already within a 100 mVof its final value) and the held V_(in) output of sample and hold 304.The remaining 70 nsec of the DACSTTL pulse permit DAC and erroramplifier settling. Indeed, simulations show DAC settling to 14-bitaccuracy in about 50 nsec. Flash converter 306 is tracking the erroramplifier output.

One gate delay after DACSTTL returns low FLASH2 goes high to driveFLASHCLK high and have the comparators and the encoder of flashconverter 306 latch the 7 bits encoding the quantized version of theoutput of error amplifier 312, and the falling edge of FLASH2 (28 nseclater) drops FLASHCLK low which latches the 7 bits in the CMOS latchesof LSB Latch 314. See the ninth panel of FIG. 49.

One gate delay after FLASH2 returns low LOADOP goes high to drive andLOADOP remains high for 20 nsec for the digital computation.

One gate delay after LOADOP returns low ACQUIRE goes high to drive IRQlow and perform six functions: (1) terminate the interrupt signal on theoutput bus, (2) switch the input to DAC 310 from the 7 bits in MSB Latch308 to the 7 bits 1000000 and thereby force DAC 310 back to a 0 voltoutput, (3) switch Clamp high to clamp error amplifier 312 to a 0 voltoutput, (4) put the results of the data output flip-flops onto theoutput bus, (5) switch sample and hold 304 back to the sampling mode,and (6) drive SWITCH low to throw analog switch 334 to feed the outputof sample and hold 304 to flash converter 306 instead of the output fromerror amplifier 312. Thus flash converter 306 begins tracking thevarying V_(in) output of sample and hold 304 again. ACQUIRE remains highfor 100 nsec to permit sample and hold 304 to settle in to trackingV_(in).

One gate delay after ACQUIRE returns low EOC goes high to drive CNTRLhigh, and then 20 nsec later EOC goes low to drive CNTRL low and IRQhigh to begin another conversion cycle. Note that the 20 nsec of EOChigh is also time for sample and hold 304 to settle to tracking V_(in).

The timing of operations shown in FIG. 49 have various features,including the following. (1) SWITCH simultaneously changes the 7 inputbits to DAC 310 from 1000 000 to the 7 bits in MSB Latch 308 in contrastto just letting DAC 310 follow the 7 bits being output be encoder array906 of flash converter 306; this prevents extreme output swinging (suchas if the second most significant bit switches and then shortlythereafter the most significant bit switches) and may provide a quickeroverall settling of DAC 310 despite the extra time taken to load MSBLatch 308 and switch the gates in FIG. 30. The 10 nsec delay betweenSWITCH going high and Clamp going low covers the time for Subtractor 316to complete its operation plus DAC 310 to complete the bulk of theoutput swing; thus the noise generated by digital Subtractor 316 andlarge swings of DAC 310 output subside prior to activation of erroramplifier 312 and help avoid saturation of its transistors. In contrast,if the error amplifier were continually active but with diode outputclamping in an attempt to limit transistor saturation, then the largeinput swings and noise during the 10 nsecs while DAC 310 output swingsand subtractor 316 switches may cause Zener breakdown of theemitter-base junctions of input transistors and, furthermore, the outputof the error amplifier likely would swing rapidly between its clampedextremes and thereby drive flash converter 306 wildly. The timing ofconverter 300 illustrated in FIG. 49 aggregates the digital noise fromsubtractor 318 with the rapid swings of DAC 310 in the same 10 nsecperiod during which error amplifier 312 is clamped and has inputtransistors in a very low current state.

(2) Another feature of the timing of FIG. 49 is the simultaneousswitching of sample and hold 304 from hold mode to sampling mode and thethrowing of analog switch 334 to switch the input to flash converter 306from the output of error amplifier 312 (which is simultaneously beingclamped) to the output of sample and hold 304. Both the throwing ofanalog switch 334 and the switching to sampling mode create largetransients for the input of flash converter 306, and thus theaggregation of these transients into a single time interval provides forquicker overall converter operation. The 100 nsec duration of theACQUIRE pulse plus the following 20 nsec of the EOC pulse provide sampleand hold 304 sufficient time to settle to tracking V_(in) (t); FIG. 8shows a simulation. Recall that the input amplifier 602 was groundedduring the hold mode to prevent saturation, and that with an inputbandwidth of 30 MHz the input V_(in) to amplifier 602 could haveoscillated between its extremes five or six times during the hold mode.

(3) A further feature of the timing of FIG. 49 occurs when flashconverter latches in a quantization and its encoding: converter 300 doesnot execute any other operations simultaneously and the latching byflash converter 306 happens at the end of a quiet settling period: afterthe 30 nsec of HLDSTTL or after the 70 nsec of DACSTTL following the 10nsec delay. This prevents noise generated by other operations corruptingthe accuracy of the flash conversions; in particular, the subtraction inblock 316 operation must be performed prior to the error correction inblock 318 if both carries CR1 and CR2 will be used, and thus aggregatingthe subtraction with the initial swings from DAC 310 effectively putsthe subtraction noise in an already-disturbed time interval. If bothcarries were not needed, then the subtraction could be merged with theerror correction.

A feature not explicit in the timing of FIG. 49 lies in the sequentialturning on of the output drives in buffer 320 to avoid kickback (notethe inductive bond wires from the substrate to its lead frame) andground bounce that may occur with all drivers being simultaneouslyturned on. As indicated by layout FIG. 44 the drivers are located alongthe outer edge of the silicon substrate containing converter 300, andthese drivers have data lines and an enable line that originate in area4450 and follow the edge of the substrate and thereby provide by theirpropagation delay a sequential turning on of the drivers. Note that allof this driver activity occurs at the same time that the rising edge ofACQUIRE switches sample and hold 304 to sampling mode and throws analogswitch 334; that is, the driver transients are also aggregated withother noisey operations into a common time interval.

Alternative embodiments that preserve some of the foregoing timingfeatures include using an always-on amplifier, but switching off itsinput during the 10 nsec (or more) that include the largest transientsof DAC 310. The DAC could have continually updated input bits with suchan input-switched error amplifier.

The following table summarizes the operation of converter 300 in termsof the external inputs #CS, #CONV, #OE, and #A0; the table also showsthe output IRQ:

    ______________________________________                                        #CS  #CONV    #OE    #A0  IRQ  Function                                       ______________________________________                                        1    x        x      x    0    No operation                                   0    0        x      x    x    Continuous convert mode                        0    x        0      0    x    Output 12-bits or 8 MSBs                       0    x        0      1    x    Output 4 LSBs with trailing                                                   0s                                             0    1        x      x    0    Converter in acquisition                                                      mode                                           0    x        x      x    1    Converter doing conversion                     0    x        1      x    x    High impedance output state                    ______________________________________                                    

Of course, the continuous convert mode requires a falling edge for #CSand #CONV to get started.

FIG. 50 schematically shows the power up reset (PUR) circuit 5000 withinblock 330. Circuit 5000 provides a PUR pulse to insure variouscomponents of converter 300 are put into known initial states upon powerup of converter 300. In particular, the two digital power supplies, Vddat +5 volts and Vss at -5 volts, and the two analog power supplies, Vccat +5 volts and Vee at -5 volts, may be applied in differing orders andlead to erratic behavior by partially powered-up devices. Circuit 5000operates as follows: NMOS differential pair 5001-5002 compares thevoltages at nodes 9 and 10 where the voltage at node 9 is resistor 5011,5013 division of Vcc to ground and equal to about 0.6 Vcc, and thevoltage at node 10 is resistor 5012 diode NMOS 5010 division of the sameVcc to ground. So Vcc rising from ground towards +5 volts will cause thevoltages on nodes 9 and 10 to rise. However, diode 5010 has a turn onvoltage of about 1-2 volts; so for Vcc small, the voltage at node 10will track Vcc and the voltage at node 9 will track 0.6 Vcc. Diode 5010has an on impedance that together with resistor 5012 divides Vcc toabout 0.5 Vcc at node 10; thus as Vcc increases above about 2 volts thevoltage on node 10 increases less rapidly than that on node 9, and atVcc equal to about 4 volts the voltage on node 9 surpasses that on node10. FIG. 51 illustrates the voltages at nodes 9 and 10 for a linearlyincreasing Vcc. Now with Vcc at about 2-3 volts the digital devices suchas inverters 5030-5031 and exclusive NOR gate 5040 become active (Vddconnects to Vcc through resistor 5020), and until NMOS 5001-5002 turn onboth inverters 5030-5031 will see a Vcc input and output lows toexclusive NOR 5040 and thus a high PUR.

NMOS differential pair 5001-5002 remains off until Vee has dropped belowabout -2.8 volts (four Vbe's) to turn on the current source made of NPN5050, diodes 5051, and resistor 5052. Thus two cases occur: (1) Vccrises more quickly than Vee falls and (2) Vee falls more quickly thanVcc rises. In the first case no current flows in 5050-5051 because Veeis less than -2.8 volts and nothing drives the differential pair5001-5002. Therefore, resistors 5003 and 5004 pull both nodes 11 and 12high with the rising Vcc and no current flows. This drives inverters5030 and 5031 both low giving a high at output PUR. As soon as a currentflow from Vee through the current source is established, thedifferential pair 5001-5002 switches and forces nodes 11 and 12 inopposite directions due to the differential pair action, and thisswitches PUR low.

In the second case differential pair 5001-5002 has its current source onwhile Vcc is still low, and Vcc low implies low inputs to inactiveinvertors 5030, 5031. As Vcc rises to about 2-3 volts, digital devicesactivate and NMOS 5002 conducts due to node 10 being higher than node 9.Thus node 11 is high and node 12 is low to yield a low at PUR. Then whenVcc reaches about 4 volts nodes 9 and 10 have about the same voltage andboth NMOS 5001 and NMOS 5002 conduct to have nodes 11 and 12 both lowand have exclusive NOR 5040 drive PUR high. Next, as Vcc exceeds 4volts, the voltage at node 9 exceeds the voltage at node 10, and NMOS5002 stops conducting to switch node 12 high and thus exclusive NOR 5040high and PUR low. That is, as the voltage at node 9 passes that at node10 the inverters 5030-5031 sequentially switch and generate a PUR pulse.The width of the pulse depends upon the thresholds of inverters5030-5031. The PUR pulse drives the master reset (MR) of both the cellsof timing generator 4500 and controller 4800.

Similarly, during normal operation if Vee should rise from -5 volts toabout -2.8 volts, then PUR will go high until Vee again drops below -2.8volts. Also, if Vcc drops below about 4 volts, again PUR will go high.Hence, circuit 5000 also detects power supply interruptions.

Voltage Reference

Voltage reference 326 provides a temperature stabilized referencevoltage of about 2.5 volts with a variation of at most 1 mV over atemperature range of -55 C. to +125 C. Voltage reference 326 includes abandgap generator with correction circuitry as shown in schematic FIGS.52a-b and 53. In particular, voltage reference 326 includes a standardbandgap circuit plus a correction circuit 5300; for explanation,consider the simplified version of voltage reference 326 shown in FIG.54. Operational amplifier 5402 (5202 in FIG. 52a) drives the bases ofNPN transistors 5411 and 5431 (5211-5224 in parallel and 5231-5232 inparallel, respectively, in FIG. 52b) where NPN 5411 has an emitter withseven times the area of the emitter of NPN 5431. The collectors of NPNs5411 and 5431 connect to a power supply through equal resistors 5441 and5442 (5241 and 5242 in FIG. 52b) with the inputs of amplifier 5402tapping off at the collectors. Amplifier 5402 insures that the collectorcurrents of NPNs 5411 and 5431 remain the same, and thus the differencein emitter areas implies that there is a difference in the base toemitter voltage Vbe for the NPNs. This difference voltage ΔVbe equals(kT/q)ln7 where k is Boltzmann's constant, T the absolute temperature,and q the electronic charge. ΔVbe equals about 50 mV at room temperatureand increases linearly with absolute temperature. This differencevoltage appears across resistor 5445, and the voltage Vg0 at the basesof the NPNs is given by Vg0=Vbe+(ΔVbe)2R₁ /R₂ where Vbe is thebase-emitter bias for NPN 5431, R₁ is the resistance of resistor 5446and R₂ is the resistance of resistor 5445 (resistors 5246 and 5245,respectively, in FIG. 52b). Now Vbe decreases linearly with absolutetemperature, so picking the ratio of the resistances correctly makes Vg0temperature independent, at least to first approximation. The resistivedivider made of resistors 5451-5453 (resistors 5251-5253 in FIG. 52a)steps up Vg0 to Vout which is close to 2.5 volts. The circuitry 5270 inFIGS. 52a-b suppresses power supply noise and cancels base current errorof all NPNs connected to node Vg0.

To correct the output Vout (Vref in FIG. 52b) for its approximate 6 mVvariation over the temperature range -55 C. to +125 C. (illustrated inFIG. 55), correction circuit 5300 absorbs a temperature-dependentcompensation current Icom from resistor divider 5451-5453 and therebyincreases Vout by RIcom where R is the resistance of resistor 5451 (5251in FIG. 52a). FIG. 56 shows correction circuit 5300 in simplified form.Compensating current Icom is derived from comparing ΔVbe (which variesdirectly with absolute temperature) to Vout/K, a fraction of Vout whichis almost temperature independent when compared to changes in ΔVbe.These two voltages are fed into NPN differential pair 5601-5602(5301-5302 in FIG. 53) with the NPNs sized so that Icom will be zero atthe peak temperature T_(p) where Vout would peak without Icom. FIG. 55that shows the peak temperature to be about +27 C. (roughly, roomtemperature). At T_(p) the equal collector currents, I, of NPNs5601-5602 equal the currents delivered by current sources 5611-5612(current mirrors 5311-5312 from 5313). Diodes 5621-5622 (5321-5323 inFIG. 53) insure that Icom always increases Vout. For temperatures aboveT_(p) Icom flows through diode 5621 and satisfies:

    ΔV.sub.be -Vout/K=(kT/q) ln [A(Icom+I)/(I-Icom)]+R.sub.1 (Icom+I)-R.sub.2 (I-Icom)

where A is ratio of the area of the emitter of NPN 5601 to that of NPN5602 and R₁ and R₂ are the resistances of resistors 5631 and 5632,respectively (resistors 5331 and 5332 in FIG. 53). For temperaturesbelow T_(p) Icom flows through diode 5622 and satisfies the followingequation:

    ΔV.sub.be -Vout/K=(kT/q) ln [A(Icom-I)/(I+Icom)]+R.sub.1 (Icom-I)-R.sub.2 (I+Icom)

As previously noted, A is fixed to make Icom zero at T_(p) whichtranslates to

    ΔV.sub.be -Vout/K=(kT.sub.p /q) ln A+R.sub.1 I-R.sub.2 I

for both equations. This still has R₁ and R₂ as variables, and these arepicked by making the compensation voltage generated by Icom at -55 c.and +125 C. just cancel the deviation of the uncompensated Vout from itspeak value; that is, in FIG. 55 the endpoints of the curve are pulledup. FIG. 57 shows the compensated Vout. FIG. 53 shows Vout/K to begenerated by resistor divider 5351-5352, ΔVbe by current mirroring fromVg0 driving NPN 5360 with emitter resistor 5361 to current source 5314and resistor 5362, and current mirroring from 5315 to provide the 2Icurrent source 5370 (5670 in FIG. 56).

Correction circuits 5300 and 5600 supply the compensation current Icomwithout any switching devices and thereby avoid switching noise.

Method of Fabrication

FIGS. 60-80 illustrate in cross sectional elevation view steps in afirst preferred embodiment method of integrated circuit fabrication. Themethod may be used to fabricate converter 300 and the variationsdescribed. The method provides both high performance bipolar transistorsand high packing density CMOS transistors. This permits integration ofmixed-mode analog-digital circuits without loss of performance overmultiple chip implementations. Indeed, analog circuits often requirebipolar devices due to their high transconductance, low 1/f noise, andease of matching Vbe, whereas digital circuits often require CMOSdevices due to their high packing density, high noise threshold, and lowpower dissipation. The method provides the following devices: an NPNtransistor with a beta of at least 80 and a cutoff frequency f_(T) of atleast 4 GHz and a breakdown voltage of at least 10 volts, an isolatedPNP transistor with a beta of at least 40 and an f_(T) of at least 1.0GHz, a high beta NPN transistor with a beta of at least 300, a substratePNP transistor, 5-volt NMOS and PMOS for digital circuitry, 10-bolt NMOSand PMOS for analog circuitry, an isolated poly-to-poly capacitor usingpoly oxide, and a precision laser-trimmable thin-film NiCr resistor foroptimizing circuit performance after fabrication. The power supplieswould be at -5 volts, ground, and +5 volts with the substrate at about-5 volts. The digital CMOS operates between ground and +5 volts despitethe substrate bias. FIGS. 58a-d show typical plan views of variousdevices, and FIG. 59a-h illustrate the doping profiles of variousdevices. The effective gate lengths are typically 0.9 μm and the emittersize about 1.4 μm square although other sizes are available with thesame process steps.

The method is modular so that various groups of steps may be omitted ifa circuit does not demand all of the foregoing devices; however, themethod uses only 21 mask levels to fabricate all of these devices. Inaddition, one further mask level permits inclusion of a low-noise Zenerdiode. The method, including the Zener diode fabrication, includes thefollowing steps:

(1) Begin with a <100> oriented monocrystalline silicon wafer of p-typewith resistivity in the range of 8 to 15 ohm-cm and with oxygenconcentration in the range of 30 to 36 parts per million. This level ofoxygen exceeds the room temperature solid solubility limit, and the heattreatments of steps (2) and (11) initiate deep defects sites andprecipitate oxygen in the interior of the silicon wafer. Laterprocessing steps will grow these initial deep defects into majordislocations and will also drive oxygen from the surface leaving adenuded surface zone. The dislocations and precipitated oxygen willgetter various impurities such as iron and copper introduced insubsequent processing steps, and the denuded zone provides low defectsilicon for device fabrication. These internal defects decrease thelifetimes and diffusion lengths of minority carriers deep in thesubstrate. Note that this enhances the effectiveness of the noisesuppressing buried layers 8601, 8602 and 8605 discussed below andillustrated in FIGS. 86-87.

(2) Thermally grows a silicon dioxide ("oxide") layer of thickness 5300Å on the surface of the silicon wafer. Steam oxidation (about one hourat 1050 C.) provides quicker oxidation than dry oxidation (more than 10hours at 1100 C.). Indeed, growth in oxygen for two hours at 750 C. willstabilize microclusters of oxygen precipitates and a subsequent growthin steam for one hour at 1050 C. will generate interstitial siliconwhich helps dissolve oxygen near the wafer surface to form a denudedzone for device fabrication.

(3) Spin a layer of photoresist onto the oxide coated wafer, and exposeand develop a pattern in the photoresist defining all needed N+ buriedlayers. Both types of NPN devices (regular and high beta) and both typesof PMOS devices (digital and analog) plus poly-to-poly capacitors andNiCr resistor areas will all be situated over N+ buried layers.

(4) Use the patterned photoresist as a mask to wet etch the exposedunderlying oxide with buffered HF.

(5) Strip the patterned photoresist with piranha (a sulfuric acid,hydrogen peroxide solution). This leaves the oxide coated silicon waferwith openings in the oxide layer at the locations of eventual buried N+layers.

(6) Implant arsenic ions at an energy of 80 KeV and a dose of 3×10¹⁵ions/cm² using the patterned oxide as an implant mask. The projectedrange for arsenic ions at 80 KeV is about 400-500 Å in both silicon andoxide, so the arsenic ions do not penetrate the oxide and only enter thesilicon through the openings defining the N+ buried layers.

(7) Spin another layer of photoresist onto the oxide coated wafer, andexpose and develop a pattern in the photoresist defining all needed N-buried layers. Both digital NMOS and PMOS devices plus isolated PNPdevices and Zener diodes will all be located over N-buried layers. TheHV-NMOS device can optionally be fabricated over an N-buried layer, asshown in FIG.f, but not shown in FIGS. 60-80. This layer of photoresistwill cover all openings in the underlying oxide through which thearsenic was implanted in step (6) except in the locations of digitalPMOS devices where the opening in the oxide will again be exposed.Additionally, the oxide in the N- buried layer locations will beexposed. Note that only a single oxide as being used for both N+ and N-buried layer location definition; this avoids oxide strip and regrowthsteps.

(8) Use the patterned photoresist as a mask to wet etch the exposedunderlying oxide with buffered HF. Buffered HF etches oxide much fasterthan silicon, so the exposed silicon in the digital PMOS locations willnot be significantly etched.

(9) Implant phosphorus ions at an energy of 120 KeV and a dose of 2×10¹³ions/cm² with the patterned photoresist as the implant mask. Theprojected range of phosphorus at 120 KeV in photoresist is about 2000 Åand in silicon about 1400 Å; thus the photoresist can effectively maskthe phosphorus even over the locations of oxide openings from step (4).Note that the phosphorus (peak 1400 Å) is much deeper than thepreviously implanted arsenic (peak 500 Å) in the locations for digitalPMOS devices.

(10) Strip the patterned photoresist with piranha. This leaves the oxidecoat with openings from both steps (4) and (8).

(11) Anneal the oxide coated wafer in an oxidizing atmosphere to bothgrow 2300 Å of oxide on exposed silicon (and further increase thethickness of the existing oxide coat elsewhere) and drive in theimplanted arsenic and phosphorus. The oxide grows faster on the exposedsilicon, so when the oxide is removed in step (13) a faint pattern ofthe N+ locations will appear on silicon surface. The phosphorus diffusesfaster than the arsenic, and the resulting N+ arsenic doped regionsextend down about 3 micrometers (μm) from the wafer surface and the N-phosphorus doped regions extend down about 7 μm. Note that a singledrive in diffusion for both the arsenic and the phosphorus savessignificant overall processing time in comparison with separate driveins of the arsenic and phosphorus. The oxide growth plus drive in may beperformed as follows: first, use a nitrogen atmosphere (with a littleoxygen to prevent silicon nitride formation) at 750 C. for about threehours to condense oxygen nucleation in bulk so unstable microclustersgrow into more stable precipitate centers which later attract moreoxygen and lead to large defects. Second, again in a nitrogen atmospherewith a little oxygen at 1200 C. for about three and two thirds hours,drive in the buried layer implants, denude the surface, and grow bulkdefects. Lastly, in a stream atmosphere at 950 C. for one half hour,grow the majority of the oxide.

(12) Strip the oxide with buffered HF. FIG. 60 illustrates the resultingregions in wafer 6001 for representative devices as follows: 6010 and6020 will be N+ buried layers for NPN and high beta NPN devices,respectively, 6030 will be an N- buried layer for an isolated verticalPNP, a substrate PNP does not need the buried layer, 6040 will be an N-buried layer for a Zener diode, a high voltage NMOS device does not needa buried layer, 6050 will be the N+ buried layer for a high voltage PMOSdevice, 6060 is a buried N- layer for the digital NMOS and PMOS with6070 the N+ buried layer for the digital PMOS, and 6080 will be an N+buried layer for a poly-to-poly capacitor and for a NiCr resistor. Theburied N- layer 6060 will form a pseudo-substrate for the digital CMOS:wafer 6001 will be biased at -5 volts and the analog devices (bipolarand high voltage CMOS) will operate between power rails at +5 volts and-5 volts, whereas the digital CMOS will operate between the usual 0 and+5 volts. Thus the digital CMOS needs isolation from the portion ofwafer 6001 at -5 volts. Buried layer 6060 biased at +5 volts (usual CMOSbias for N substrate) provides this isolation by forming a reversedbiased junction with the remainder of wafer 6001. Hence, switching noiseelectrons generated by the digital CMOS will be contained in N-layer6060 and away from the analog devices by the 10-volt barrier at thejunction with the P-wafer at -5 volts.

(13) Spin a 1.5 μm thick layer of photoresist onto bare wafer 6001, andexpose and develop a pattern in the photoresist defining all needed P+buried layers and also P+ channel stops. The buried P+ locations may bealigned to the pattern of the N+ buried layer locations. Isolated PNPdevices, substrate contacts, Zener diodes, and both high voltage anddigital NMOS devices will all be located over P+ buried layers.

(14) Implant boron ions at an energy of 120 KeV and dose of 1×10¹⁴ions/cm² using the patterned photoresist as the implant mask. The boronhas a projected range of about 3500 Å in silicon and 5000 Å inphotoresist. Strip the photoresist with piranha; FIG. 61 shows theresulting cross section with representative device locations. Inparticular, P+ buried channel stop regions 6110 will eventually be underrecessed isolation oxide regions, P+ buried layer 6120 will be thesubcollector for the isolated vertical PNP device, P+ buried layer 6130will be part of the surface collector contact for the substrate PNPdevice, P+ buried layer 6140 will be part of the anode structure of theZener diode, and P+ buried layers 6150 and 6160 will underlie the highvoltage analog and digital NMOS devices, respectively. Note that P+buried layers 6120, 6140, and 6160 lie completely within N- buriedlayers 6030, 6040, and 6060, respectively, which act as pseudo N-substrates. Later oxide isolation makes this structure essentiallybecome an N- substrate on P-wafer 6001 and yields isolated circuits andtrue complementary devices from a triple buried layer structure. Theimplanted boron will be driven in to a depth of about 2.5 μm during theepitaxial deposition of step (15), so there is no separate drive inanneal.

(15) Etchback about 2000 Å of implanted wafer 6001 in HCl at 1175 C. (2minutes) in preparation for epitaxial deposition; this etchback must belimited to avoid removing a significant amount of the implanted boron.Epitaxially deposit in situ arsenic-doped silicon layer 6210 ofthickness 1.7 μm onto implanted wafer 6001 by thermal decomposition(˜1080 C.) of dichlorsilane plus arsine. The arsenic doping level is setto yield a resistivity of 0.8 ohm-cm (roughly 8×10¹⁵ atoms/cm³) forlayer 6210. This combination of thickness and resistivity provides thecorrect performance of the NPN devices in terms of breakdown and Earlyvoltages plus also permits counter doping to provide P wells in theepilayer 6210. The P wells need to be generated with a very low thermalbudget, so epilayer 6210 must be thin. The epitaxial depositiontemperature also drives in the boron implanted in step (14). FIG. 62illustrates the epilayer 6210 on wafer 6001. Further, the avoidance ofdrive ins (low thermal budget) and a shallow emitter permit such a thinepilayer due to the lessening of dopants diffusing up from the buriedlayers (subcollectors) to narrow the active collectors. Indeed, theepilayer thickness and doping relate to Early voltage andemitter-collector breakdown so that the following can be achieved forthe NPN devices:

    __________________________________________________________________________    Operating Voltage                                                                       Epi Deposition Thickness                                                                   Epi Doping/cm.sup.3                                                                    BV.sub.ceo                                                                         Early Voltage                                                                         β                           __________________________________________________________________________    10 V      1.5-1.8 μm                                                                              5 × 10.sup.15 - 1 × 10.sup.16                                              11-13 V                                                                            35-55 V 90-150                           20 V      2.2-2.8 μm                                                                              2 × 10.sup.15 - 5 × 10.sup.15                                              20-24 V                                                                            40-65 V 90-150                           __________________________________________________________________________

The specific details below are for the 10-volt process. Note that theproduct of beta times early-voltage is at least 5000, and about 6000 istypically achieved.

(16) Thermally grow a pad oxide of thickness 625 Å of epilayer 6210 insteam at approximately 900 C.; this consumes about 300 Å of epilayer6210. This pad oxide will provide stress relief for the silicon nitride("nitride") oxidation mask during the subsequent recessed localoxidation of the silicon (LOCOS) to create recessed isolation oxideregions.

(17) Deposit by LPCVD a 1300 Å thick layer of nitride on the pad oxide.

(18) Spin a 1.5 μm thick layer of photoresist onto the nitride/oxidecoated wafer 6001, and expose and develop a pattern in the photoresistdefining all recessed isolation oxide locations.

(19) Plasma etch the nitride, pad oxide, plus underlying silicon usingthe patterned photoresist as the etch mask. The nitride and pad oxideare relatively thin, so an isotropic etch would suffice for the initialstages of the plasma etch, and a mixture of SF₆ and O₂ gives arelatively anisotropic etch of the silicon. Etch about half way throughepilayer 6210, that is, to a depth of about 0.65-0.7 μm.

(20) Strip the photoresist with piranha. This leaves trenches in thewafer 6001 with the patterned nitride and pad oxide coating the tops ofthe mesas between the trenches.

(21) Oxidize the exposed silicon trenches in an oxygen atmosphere at 975C. and a pressure of 25 atmospheres for 25 minutes to grow oxide to athickness of 1.5-1.7 μm. The nitride protects the mesa tops fromoxidation, but oxide grows laterally under the edges of the nitride toform "bird's head" bulges which will be eliminated in step (22). In thetrenches the oxidation consumes the remaining vertical portion ofepilayer 6210 and reaches to buried P+ channel stop regions 6110 or theP+ buried layers 6120, 6150, and 6160 and the N+ buried layers 6010,6020, 6050, and 6070. Note that the relatively thin epilayer 6210permits the oxidation to consume the epilayer in the trenches withoutcreating excessive bird's head or overrunning a low thermal budget.Also, the thin epilayer 6210 permits narrow recessed isolation oxideregions for close packing of devices, especially among the NPN deviceswhich need isolated collectors. The isolation oxide extends above thesilicon surface and this permits later planarization to avoid touchingthe mesa silicon. This also permits pad oxide overetch in step (31) toremove the "bird's beak" without recessing the isolation oxide top;consequently, the gate width of MOS devices increases. Note that thedeposited epilayer thickness was about 1.7 μ m but up diffusion of theburied layers decreases this to about 1.3 μm if the edge of the buriedlayer is taken to be where the dopant concentration exceeds the originalepilayer concentration by a factor of 10; that is, about 1×10¹⁷. Theisolation oxide grows down to overlap the buried layers and therebyperform its isolation function.

(22) Spin on planarizing photoresist to a thickness of 1.5 μm; thephotoresist covers the irregular surface created by the oxidation ofstep (21) but has an essentially planar top surface. Etch back thephotoresist plus the bird's head oxide bulges with a plasma etch of CHF₃and O₂. This removes all of the photoresist and approximately planarizesthe surface.

(23) Strip the nitride with hot H₃ PO₄. FIG. 63 shows the resultingstructures in wafer 6001 with the recessed isolation oxide regions 6310and 6320; the pad oxide is too thin to show on the drawings but remainsas a deterrent to channeling in the implants of steps (25), (26), (29),(30), and (33). The 6310 isolation regions have underlying P+ channelstop buried regions and separate two N type buried layers.

(24) Spin a 1.5 μm thick layer of photoresist onto wafer 6001, andexpose and develop a pattern in the photoresist defining the N wellsneeded for both analog and digital PMOS devices. (N-well is primarilyjust epilayer 6210; this step is for surface doping to form the buriedchannel of proper V_(tp).)

(25) Implant threshold adjusting boron ions at 30 KeV and a dose of2.3×10¹² ions/cm² using the patterned photoresist from step (24) as theimplant mask. This boron dose will set the PMOS device thresholdvoltages to about -1.0 volt. Note that the projected range for 30 KeVboron ions is about 1000 Å in silicon.

(26) Implant N well phosphorus ions at 160 KeV and a dose of 1.5×10¹²ions/cm² using the same patterned photoresist as the boron implant ofstep (25). The projected range of 160 KeV phosphorus is about 2200 Å;thus the phosphorus implant lies beneath the boron threshold adjustmentimplant but the phosphorus implant remains close to the surface. Recallthat epilayer 6210 had a deposited thickness of about 1.7 μm thick andan arsenic concentration of about 8×10¹⁵ atoms/cm³, but over the N+buried layers the epilayer 6210 deposition itself and other heattreatments caused updiffusion so the effective epilayer thickness isabout 1.2 μm. After the boron and phosphorus implants, the net donorconcentration at a depth of about 2000 Å is 1.5×10¹⁶ atoms/cm³ and at adepth of about 1000 Å the boron has converted the doping to a netacceptor concentration of about at most 1×10¹⁶ atoms/cm³. The PNjunction formed at a depth of about 1500 Å has a depletion regionextending to the wafer surface, and the PMOS devices will be buriedchannel type devices. The high voltage PMOS will almost be a surfacechannel device due to the two gate oxidations, and V_(tp) is fairlyhigh. Indeed, the N wells have an overall retrograde doping (increasingdonor concentration with depth) down to the N+ buried layer peak despitethe bump from this phosphorus implant, see FIG. 59d. In general,retrograde doping reduces latchup and snapback parasitics by providinghigh conductivity wells in spite of the low surface doping required forproper MOS thresholds. The buried N+ layers below the N wells furtherreduces latchup and snapback by providing very high conductivityregions. The well anneal of step (35) will spread out the implants, butthe digital PMOS devices will remain buried channel devices and the highvoltage analog PMOS devices will be almost surface channel devices.

(27) Strip the patterned photoresist with piranha. FIG. 64 shows theresulting structure with N wells 6450 for analog PMOS devices and Nwells 6470 for digital PMOS devices.

(28) Spin a 1.5 μm thick layer of photoresist onto wafer 6001, andexpose and develop a pattern in the photoresist defining the P wellsneeded for both analog and digital NMOS devices, the Zener diodes(optional), and also the collector for the isolated PNP and a portion ofthe collector contact structure for the substrate PNP.

(29) Implant threshold adjusting boron ions at 50 KeV and a dose of2.8×10¹² ions/cm² using the patterned photoresist from step (28) as theimplant mask. This boron dose will set the digital NMOS device thresholdvoltages to about +0.65 volt. The projected range for 50 KeV boron ionsis about 1600 Å in silicon and the dose suffices to convert the upper1000 Å of epilayer 6210 to P type.

(30) Implant P-well, P double-charged boron ions at 125 KV and a dose of2.7×10¹² ions/cm² using the same patterned photoresist as the boronthreshold adjustment implant of step (29). The projected range of 250KeV double-charged boron is about 6500 Å; this dose suffices to convertepilayer 6210 to P type despite the original arsenic concentration ofabout 8×10¹⁵ atoms/cm³. During the well anneal of step (35) boron fromP+ buried layers 6120, 6130, 6150, 6160 will diffuse slightly upwardsand meet the spreading implanted boron to change all of epilayer 6210 toP type with the net donor concentration averaging about 4×10¹⁶ atoms/cm³and peaking at the original implant depth of about 6500 Å. The P wellsincluding P+ buried layers have an effectively retrograde doping, butless drain capacitance than implanted-only retrograde well; see FIGS.59c, 59e, and 59f for doping profiles of the completed devices. Theburied P+ layers below the P wells further reduces latchup and snapbackby providing high conductivity regions. Also, the vertical PNP deviceswill use this P well as its collector, so the higher doping levels willlessen the resistivity between the P+ subcollector 6120 and thecollector contact 7526 to be formed later.

(31) Strip the patterned photoresist with piranha. FIG. 65 illustratesthe P collector 6520 of the isolated PNP, the collector contact portion6530 of substrate PNP, P well 6540 (optional) for the Zener diodes, andP wells 6550 and 6560 of analog and digital NMOS devices, respectively.

(32) Spin a 1.5 μm thick layer of photoresist onto wafer 6001, andexpose and develop a pattern in the photoresist defining the Zener diodelocation.

(33) Implant boron ions at 160 KeV and a dose of 1.2×10¹⁴ ions/cm² usingthe patterned photoresist from step (32) as the implant mask. Theprojected range of 160 KeV boron is about 4500 Å. The boron dosesuffices to convert the central portion of epilayer 6210 to P type witha doping concentration of roughly 1×10¹⁸ atoms/cm³. FIG. 59h shows thedoping profile for the completed Zener diode.

(34) Strip the patterned photoresist with piranha; FIG. 66 shows theresulting converted portion 6640 of epilayer 6210 for the Zener diode.

(35) Anneal wafer 6001 in a nitrogen atmosphere for 30 minutes at 950 C.This activates the implanted dopants and causes some diffusion,especially of the boron.

(36) Strip the pad oxide with an HF etch plus overetch to remove most ofthe bird's beak from the oxidation of step (21). This effectivelyincreases the width of the silicon mesas between the isolation oxides.After a cleanup, thermally grow 185 Å thick gate oxide on the exposedsilicon of wafer 6001 in an oxygen atmosphere at 920 C.; of course, theisolation oxides 6310 and 6320 also increase slightly in thickness.

(37) Deposit 5500 Å thick undoped polysilicon on the oxidized wafer 6001with LPCVD by silane decomposition.

(38) Spin a 1.5 μm thick layer of photoresist onto the polysiliconcovered wafer and expose and develop a pattern in the photoresistdefining the deep N+ contacts to N+ buried layers 6010 and 6020 and N-buried layer 6030. Recessed isolation oxide surrounds these deep N+contact locations, so the contacts will self-align with largephotoresist openings.

(39) Plasma etch openings in the layer of undoped polysilicon with SF₆and O₂ using the patterned photoresist as an etch mask. This etchselective etches polysilicon and effectively stops on the 185 Å oxide;see FIG. 67 showing polysilicon 6710.

(40) Strip the patterned photoresist with piranha.

(41) Wet etch (HF) the 185 Å oxide exposed by the openings in undopedpolysilicon layer 6710 formed in step (39). That is, aperturedpolysilicon 6710 forms the etch mask, so the exposed portions ofrecessed isolation oxide will also be etched, but only a few hundred Åwill be lost. FIG. 67 shows the openings 6910, 6920, and 6930 throughboth polysilicon layer 6710 and the 185 Å oxide.

(42) Dope apertured polysilicon 6710 and the silicon exposed throughopenings 6910, 6920, and 6930 with phosphorus by decomposing POCl₃ onthe surface at 890° C. The resulting resistivity of the dopedpolysilicon is about 11 ohms/square, and the upper portion of theexposed silicon dopes to N+. The deep N+ contact regions have a carrierconcentration of greater than 1×10²⁰ /cm³. This doping of both thepolysilicon layer and the deep N+ contact regions with the same stepeliminates a separate diffusion or implant.

(43) Spin a 1.5 μm thick layer of photoresist onto apertured-polysiliconcoated wafer 6001, and expose and develop a pattern in the photoresistdefining the digital NMOS and PMOS device gates and interconnectionlines plus the bottom plates of the poly-to-poly capacitors pluscovering the exposed silicon in openings 6910, 6920, and 6930. Becauseopenings 6910, 6920, and 6930 were larger than the portions of wafer6001 doped, the photoresist can be smaller than the openings and therebynot cover any of the adjacent polysilicon. The gates may have nominallengths of 1.4 μm as drawn but effective lengths of 0.9 μm.

(44) Plasma etch the polysilicon with SF₆ and O₂ or HBr and Cl₂ usingthe patterned photoresist as an etch mask. This etch selective etchespolysilicon and effectively stops on oxide, so an overetch to guaranteeremoval of polysilicon will also not etch significantly down into wafer6001.

(45) Strip the patterned photoresist with piranha, and strip the exposedgate oxide with HF leaving just gate oxide 6810 under the gates 6860 and6870 and the lower capacitor plate 6880 formed from polysilicon 6710 instep (44).

(46) Thermally oxidize patterned-polysilicon-coated wafer 6001 in anoxygen atmosphere at 920 C to grow a second gate oxide of thickness 300Å on the exposed silicon. Note that the exposed surfaces of thepatterned doped polysilicon from step (44) oxidize much more rapidlythan the silicon of wafer 6001 due to the heavy doping of thepolysilicon, and an oxide of thickness 900 Å forms on the polysilicon.FIG. 68 illustrates the resulting structure on wafer 6001 including 185Å first gate oxide 6810 under digital NMOS and PMOS gates 6860 and 6870,respectively, 300 Å second gate oxide 6820 on the wafer surface, and 900Å oxide 6830 on the surface of polysilicon gates 6860 and 6870 andpolysilicon lower plate 6880 of the poly-to-poly capacitor.

(47) Deposit a second layer of 5500 Å thick undoped polysilicon on wafer6001 with LPCVD using silane decomposition. See FIG. 69 showing secondpolysilicon layer 6950. Note that steps (38)-(41) could have beenomitted above and inserted here using polysilicon 6950 in place ofpolysilicon 6710.

(48) Dope second polysilicon layer 6950 with phosphorus by decomposingPOCl₃ at 890 C. The resulting resistivity of the doped polysilicon isabout 11 ohms/square.

(49) Spin a 1.5 μm thick layer of photoresist onto wafer 6001, andexpose and develop a pattern in the photoresist defining the analog NMOSand PMOS device gates and interconnection lines plus the top plates ofthe poly-to-poly capacitors. The gates have nominal lengths of 2.0 μm.

(50) Plasma etch the polysilicon with SF₆ and O₂ or HBr and Cl₂ usingthe patterned photoresist as an etch mask. This selectively etchespolysilicon and effectively stops on oxide, so an overetch to guaranteeremoval polysilicon will also not etch significantly down into wafer6001; however, the 300 Å second gate oxide 6820 not protected by thepatterned photoresist plus polysilicon will be partially removed, andthe 900 Å oxide 6830 on first polysilicon will be slightly thinned.

(51) Strip the patterned photoresist with piranha. FIG. 70 shows theresulting analog NMOS and PMOS gates 7050 and 7056, respectively, on 300Å second gate oxide 6820 and poly-to-poly capacitor with top plate 7080separated from lower plate 6880 by 900 Å oxide 6830. FIG. 70 also showsdeep N+ contacts 7010, 7020, and 7030 that were formed in step (42). Ifsteps (38)-(41) had been moved to follow step (47), then the doping ofstep (48) would form deep N+ contacts 7010, 7020, and 7030.

(52) Spin a 1.5 μm thick layer of photoresist onto wafer 6001, andexpose and develop a pattern in the photoresist defining the baselocations for the isolated PNP devices and also the lightly doped drainextensions of the analog NMOS devices.

(53) Implant phosphorus ions at 160 KeV and a dose of 5.0×10¹³ ions/cm²using the patterned photoresist from step (52) as the implant mask. Theprojected range of 160 KeV phosphorus is about 2200 Å. The phosphorusdose suffices to convert the upper portion of P well 6520 to N type witha doping concentration of roughly 2×10¹⁸ atoms/cm³. Recall that P well6520 has retrograde boron doping, so the remaining P type lower portionof P well 6520, which will form the active collector of the isolatedPNP, will have retrograde doping.

(54) Strip the patterned photoresist with piranha.

(55) Thermally grow a thin (300 Å) mesa oxide on the exposed surfaces ofwafer 6001 plus on the exposed surfaces of patterned second polysilicon;this oxide passivates the sidewalls of gates 7050 and 7056. The oxidegrowth also enhances the thickness of the other oxides. FIG. 71 showsmesa oxide 7190 and sidewall oxide 7170 plus the converted portion 7120of P well 6520 and the drain extension 7150 in P well 6550.

(56) Spin a 1.5 μm thick layer of photoresist onto wafer 6001, andexpose and develop a pattern in the photoresist defining the N+source/drains needed for the digital NMOS devices, the sources neededfor analog NMOS devices, and the N well contacts in both analog anddigital PMOS devices, and optionally the deep N+.

(57) Implant phosphorus ions at 100 KeV and a dose of 1.0×10¹⁴ ions/cm²using the patterned photoresist from step (56) as the implant mask. Thisphosphorus dose will form a deeper and less heavily doped peripheralportion of the source/drains and provide some doping gradient to lessenthe maximum electric fields. Note that the projected range for 100 KeVphosphorus ions is about 1200 Å in silicon and easily penetrates mesaoxide 7190.

(58) Implant arsenic ions at 100 KeV and a dose of 5.0×10¹⁵ ions/cm²using the same patterned photoresist as the phosphorus implant of step(57); this forms the more heavily doped shallower portion of thesource/drains. The projected range of 100 KeV arsenic is about 500-600 Åin silicon and oxide. Thus the arsenic implant lies near the surface,and the net donor concentration near the surface will be about 1.5×10²⁰atoms/cm³ and at a depth of about 1000 Å the net donor concentrationwill be about 1.3×10²⁰ atoms/cm³ at the end of the processing.

(59) Strip the patterned photoresist by ashing (oxygen burnoff) andpiranha. FIG. 72 shows the resulting structure with N+ sources 7250 foranalog NMOS devices, N+ well contacts 7256 and 7270 for analog anddigital PMOS devices, and N+ source/drains 7260 for digital NMOSdevices.

(60) Spin a 1.5 μm thick layer of photoresist onto wafer 6001, andexpose and develop a pattern in the photoresist defining the bases forthe high beta NPN devices.

(61) Implant boron ions at 125 KeV and a dose of 6.0×10¹² ions/cm² usingthe patterned photoresist from step (60) as the implant mask. This borondose will form a deeper portion of the bases. Note that the projectedrange for 125 KeV boron ions is about 4000 Å in silicon and easilypenetrates mesa oxide 7190 but does not extend to the bottom of N layer6210 which will form the active collector of the high beta NPN.

(62) Implant boron ions at 30 KeV and a dose of 2.0×10¹² ions/cm² usingthe same patterned photoresist as the boron implant of step (61); thisforms the shallower portion of the base. The projected range of 30 KeVboron is about 1000 Å in silicon and oxide; the resulting doping levelin the active base region averages about 1.5×10¹⁷ atoms/cm³ at a depthof about 0.4 μm. Thus the base will have a fairly small dose very deep,much deeper than the emitter to be formed, so the active electricalcharge of the base will be formed predominantly with the implant dosecontrol of the implanter and avoid heavy compensation by the emitter aswith diffused base devices. This increases the uniformity and decreasesthe variability of the high beta NPNs from lot to lot and even within adie; indeed, matches within 1-2% are obtained. The shallow boron implantprecludes inversion around the emitter but is totally compensated withinthe emitter and does not contribute a base electrical charge. This alsopermits independent tailoring of the radiation hardness of the devices.FIG. 59b shows the doping profile.

(63) Strip the patterned photoresist with piranha.

(64) Deposit 2200 Å thick borosilicate glass ("BSG") with CVD byreaction of silane, nitrous oxide, nitrogen, and diborane to yield aglass with boron in the range of 0.5% by weight. Alternately, undopedCVD oxide could be used. The BSG deposits upon the preexisting oxidesand brings the total oxide (silicon dioxide plus BSG) thickness on themesas to about 2500 Å. FIG. 73 shows the resulting H base (P type basefor high beta NPN) 7320 in N layer 6210 and deposited BSG layer 7310;note that oxide 7190 does not appear separate from BSG 7310. The oxidethickness must be uniform because the active base for the standard NPNdevices is implanted through the oxide in step (66) and thus baseimplant depth and device characteristics uniformity depend upon oxidethickness. The deposition of BSG along with the underlying thermal oxidehas a uniformity of about 0.3% of sigma.

(65) Spin a 1.5 μm thick layer of photoresist onto wafer 6001, andexpose and develop a pattern in the photoresist defining the bases forthe standard NPN devices.

(66) Implant boron ions at 130 KeV and a dose of 4.7×10¹³ ions/cm² usingthe patterned photoresist from step (65) as the implant mask. Note thatthe projected range for 130 KeV boron ions is about 4000 Å in siliconand oxide, so after penetrating the 2500 Å thick oxide 7310 the borontravels about 1500 Å into the silicon. Note that the dose exceeds thedose of the high beta NPN, so the standard NPN has a shallower and moreconductive base than the high beta NPN. Also, implanting through oxide7310 insures that crystal damage due to the implant extends to thesilicon surface and thus later annealing and crystal regrowth proceedsfrom the bulk rather than from a surface layer. FIG. 74 illustrates base7410.

(67) Strip the patterned photoresist with piranha.

(68) Anneal wafer 6001 to activate the implants and regrow damagedcrystal at 950 C. in a nitrogen atmosphere for 60 minutes.

(69) Spin a 1.5 μm thick layer of photoresist onto wafer 6001, andexpose and develop a pattern in the photoresist defining the locationsof P+ source/drains of both the analog and the digital PMOS devices, Pwell contacts for both the analog and digital NMOS devices, basecontacts for both standard and high beta NPN devices, collector contactsand emitters for both isolated and substrate PNP devices, and anodecontact for Zener diodes.

(70) Implant boron ions at 100 KeV and a dose of 1.2×10¹⁵ ions/cm² usingthe patterned photoresist from step (69) as the implant mask. Note thatthe projected range for 100 KeV boron ions is about 3000 Å in siliconand oxide with a projected straggle of about 600 Å. Thus the peak of theimplant lies near the surface of the silicon under the 2500 Å thickoxide 7310, and high concentrations of boron extend a few hundred Å intothe silicon. This implant also could be used to form substrate resistorswith resistivities of about 100 ohms/square. Note that the implant ofstep (66) which forms the bases for the NPN devices also could be usedto form substrate resistors with resistivities of about 1000 ohms/squareand the implant of steps (60-62) for bases of high beta NPN devicesleads to substrate resistors of about 3000 ohms/square. In contrast, theNiCr resistors made in steps (87-89) and which are laser trimmable formresistors of about 200 ohms/square, and resistors made from the dopedpolysilicon layers have resistivities of about 12 ohms/square. Thisindicates that the first preferred embodiment method has a variety ofresistivities for resistor fabrication.

(71) Strip the patterned photoresist with piranha. FIG. 75 shows NPNbase contact 7510, high beta NPN base contact 7520, isolated PNP emitter7524 and collector contact 7526, substrate PNP emitter 7530 andcollector contact 7532, Zener diode anode contact 7540, analog NMOS wellcontact 7550, analog PMOS source/drains 7556, digital NMOS well contact7560, and digital PMOS source/drains 7570.

(72) Deposit 7800 Å thick borophosphosilicate glass ("BPSG") by CVDusing silane, nitrous oxide, nitrogen, phosphine, and diborane to yield2-3% boron and 3.5-4.5% phosphorus by weight. The BPSG deposits upon thepreexisting oxides and brings the total oxide (silicon dioxide plus BSGplus BPSG) thickness on the mesas to about 1 μm, and this oxide iscalled the Field Oxide. The BPSG over planar areas has a thicknessvariation of only about 0.3% of sigma, so the total oxide also has highthickness uniformity.

(73) Densify the BPSG of step (72) in steam at 800 C. for 20 minutes tostabilize the boron and phosphorus dopants. Theoretically, thisdensification uses the catalytic effect of hydrogen and rapid diffusionof steam to drive the boron and phosphorus dopants to bind to oxygen inthe silicon dioxide and thus lessens the outdiffusion of dopants duringlater processing. That is, the boron and phosphorus in the as-depositedBPSG are primarily elemental, and the steam densification oxidizes theboron and phosphorus. Indeed, the diffusion of elemental dopants fromBPSG into the NiCr or other thin-film resistors to be formed laterdisrupts the resistor stability and degrades the capability of targetingthe final value of resistivity. Experimentally, steam densified BPSGreleased less than about 2×10¹⁹ /cm³ boron into a NiCr thin film,whereas dry densified BPSG released at least about 1×10²⁰ /cm³ boroninto a NiCr thin film. The bonding of the boron and phosphorus to oxygencan be detected, at least in the upper portions of the BPSG layer, byXPS (Xray photospectrometry), FTIR (Fourier transform infrared), or SIMS(secondary ion mass spectroscopy). The densification should convert mostof the boron and phosphorus to oxygen-bonded form.

However, steam densification degrades NPN performance, possibly by basegrading out to decrease the Early voltage and F_(T) or by dopantsegregation at the emitter periphery. Thus densification should be asshort as possible and at as low a temperature as possible and stillstabilize the dopants. FIG. 97 shows a time-temperature trade-off forthe steam densification. Of course, the limits could be shifteddepending upon the BPSG composition and the resistor and NPN tolerances.Furthermore, steam densification appears to degrade NMOS hot electronperformance if the boron percentage in the BPSG is high and thephosphorus percentage low; whereas if the phosphorus percentage is highand the boron percentage is low, then little degradation occurs. Thusconfine the boron to the range of 1% to 3% and have the phosphoruspercentage at least 1 higher than the boron percentage. For example,2.25% boron and 4.5% phosphorus yields good overall results. This steamdensification also increases the adherence of the TiW metal deposited instep (91) to the BPSG. Adherence problems for dry densified BPSG mayalso arise from the out diffusion of dopants.

(74) Spin a 1.5 μm thick layer of photoresist onto wafer 6001, andexpose and develop a pattern in the photoresist defining the contacts toactive regions of all devices and also to any substrate resistors.

(75) Plasma etch the oxide with CHF₃ and O₂ using the patternedphotoresist as an etch mask and with endpoint detection. Note that theoxide has various thicknesses, although each of the thicknesses is quiteuniform: the deposited oxide is 2150 Å thick, and the BPSG is 7800 Åthick. If the thermal oxide is 350 Å thick in the emitter area, then thethermal oxide would be 2000 Å thick over the Deep N+ collector (anadditional 1650 Å), and the thermal oxide would be 1200 Å thick over thefirst polysilicon gates (an additional 850 Å). Note that the oxide overDeep N+ is only 1000 Å when Deep N+ is doped by the Poly 2 doping cycle.This etch selective etches oxide at a rate more than nine times that ofsilicon, but to clear the oxide over the collector, with endpointdetection etch stop, leads to the removal of 200-300 Å of silicon in theemitter area. This removal is tolerably small because the oxides areuniform and thus require only a minimal overetch. Note that a bufferedHF etch generally has better selectively than plasma etches and does notcreate the crystal damage due to high energy ion impacts of a plasma,but wet etches generally cannot achieve the small geometries for highdigital device packing, especially through thick (1 μm) oxides.

(76) Strip the patterned photoresist with piranha; FIG. 76 shows theresulting BPSG 7610 plus BSG plus thermal oxide with smooth topographyplus apertures for contacts.

(77) Spin a 1.5 μm thick layer of photoresist onto wafer 6001, andexpose and develop a pattern in the photoresist defining the locationsof N++ which includes the NPN (both standard and high beta) emitters,the tops of deep N+ contacts 7010, 7020, and 7030, the base contact PNPs(both isolated and substrate), the Zener diode cathode, and the contactto the drain of the analog NMOS devices.

(78) Implant arsenic ions at 80 KeV and a dose of 1.0×10¹⁶ ions/cm²using the patterned photoresist from step (77) plus the exposedapertured BPSG as the implant mask. Note that the projected range for 80KeV arsenic ions is about 500 Å in silicon. The N++ emitters formed arecalled "washed emitters" and are the same size as and self-aligned tothe contact apertures in BPSG 7610 created in step (75). N++ also formsenhanced contact regions to other N-type regions like the N base of thePNP transistors. The deep N+ contacts, which cost no diffusion ordeposition step, came efficiently at the polysilicon doping step. ThisN++ arsenic implant damages the surface of wafer 6001 and the resultantdefects enhance the diffusivity of the phosphorus previously depositedduring the polysilicon doping step. Thus shorter and lower temperatureanneals of the base and emitter implants may be used and still diffusethe phosphorus down to the N+ buried layers 6010 and 6020 plus N buriedlayer 6030. The implanted emitter self-aligns to the contacts aperturesfrom step (75) rather than being nested in conventional analogfabrication. Thus the washed emitter can be the same size as the minimumcontact aperture provided by the lithography used and is much smallerthan a nested emitter. FIG. 77 shows the N++ implanted regions 7710 (NPNemitter), 7715 (NPN collector contact), 7720 (high beta NPN emitter),7725 (high beta NPN collector contact), 7730 (isolated PNP basecontact), 7735 (contact to buried layer 6030), 7737 (base contact forsubstrate PNP), 7740 (Zener cathode), and 7750 (contact to drain of highvoltage NMOS).

(79) Strip the patterned photoresist by ashing plus piranha.

(80) Deposit 200 Å thick cap oxide with CVD by reaction of silane,nitrous oxide, and nitrogen at 400 C. The oxide deposits upon both theexposed silicon in the apertures formed during step (75) and thepreexisting oxides (BPSG 7610 on mesa oxide 7310). The cap oxideprovides a barrier against autodoping during the arsenic implantactivation anneal to follow in step (81). Without cap oxide, dopantswould diffuse out of the BPSG (which is about 2.25% boron oxide and 4.5%phosphorus oxide) and into the exposed silicon.

(81) Anneal coated wafer 6001 at 1,000 C. in a nitrogen atmosphere for12 minutes (general 950-1050 C. for 5-30 min). This anneal activates anddiffuses the arsenic implants of step (78) to a depth of 0.3 μm plusflows the BPSG 7610 to smooth out the corners of the apertures etched instep (75) and over poly lines.

(82) Etch the cap oxide with buffered HF; this opens the bottoms of theapertures in BPSG 7610 and mesa oxide 7310. Note that the cap oxide(deposition in step (80) and removal in this step) could be omitted ifautodoping during the anneal to activate the emitter implants does notpush device characteristics out of an acceptable range.

(83) Sputter deposit a 280 Å thick layer of platinum onto wafer 6001.

(84) Sinter platinum-covered wafer 6001 in a nitrogen atmosphere at 450C. for 50 minutes. The platinum which deposited upon the silicon exposedby the apertures of step (75) reacts with the silicon to form platinumsilicide (PtSi), whereas the platinum which deposited upon the BPSG doesnot react because silicon, phosphorus, and boron are all moreelectropositive than platinum and will not be reduced by the platinum.Note that PtSi forms on both P type and N type silicon and both singlecrystal silicon and polysilicon, so all contacts to silicon will have aPtSi interface. PtSi has a low resistivity of 6-8 ohms/square for a thin(<500 Å) layer, and a low barrier to P-type silicon.

(85) Strip the unreacted platinum while leaving the PtSi with a wet etchusing aqua regia (HCl plus HNO₃) which dissolves platinum by formingsoluble platinum chlorides.

(86) Spin a 1.2 μm thick layer of photoresist onto wafer 6001, andexpose and develop a pattern in the photoresist defining the locationsfor nickel chromium (NiCr) thin film resistors on top of BPSG 7610.

(87) Wet etch, with buffered HF, the surface of the oxide (BPSG 7610)exposed through the openings in the patterned photoresist to slightlyundercut the photoresist. The undercut insures that the subsequentdeposited NiCr does not build up at the vertical photoresist edges ofthe exposed oxide and prevent a clean lift-off.

(88) Sputter deposit a 100 Å thick layer of NiCr (60% Cr and 40% Ni)onto photoresist covered coated wafer 6001. This NiCr film is so thinthat it does not cover the sidewalls of the openings in the patternedphotoresist but rather just covers horizontal surfaces; namely, theexposed BPSG in the photoresist openings and the top surface of thephotoresist.

(89) Lift-off the patterned photoresist by dissolving it in acetone,then methanol, and deionized water. This also lifts off the NiCr thatdeposited upon the top surface of the photoresist but does not affectthe NiCr deposited upon BPSG 7610.

(90) Strip any remaining patterned photoresist with organic solvent suchas AZ300T. FIG. 78 shows PtSi interfaces 7805-7882 and NiCr resistors7890.

(91) Sputter deposit a 1700 Å thick layer of titanium tungsten (TiWwhich is basically tungsten with about 10% titanium added for adhesion)and then sputter deposit a 6,000 Å thick layer of copper siliconaluminum (about 1% copper and 1/2% silicon with the copper added tosuppress hillocking and the silicon about the saturation limit). Thesetwo layers form the first metal level and may include localinterconnections. Adhesion is also promoted by the previous steamdensification of BPSG 7610.

(92) Spin on photoresist and expose and develop it to define locationsover the PtSi interfaces and contacts to the NiCr resistors.

(93) Plasma etch the copper silicon aluminum with Cl₂ plus BCl₃ and CHF₃using the patterned photoresist as the etch mask. This plasma etchpermits high packing density because it avoids the undercut and linewidth decrease of wet etches. The plasma etch proceeds very slowly inTiW and the etch is terminated before penetration of the TiW. Thus theTiW effectively protects the underlying thin NiCr from plasma etchdamage and subsequent change of resistivity.

(94) Strip the patterned photoresist with organic solvent followed byoxygen plasma.

(95) Wet etch the exposed TiW with EDTA plus H₂ O₂ which selectivelystops etching at NiCr, BPSG, and aluminum. Indeed, the copper siliconaluminum remaining from the plasma etch of step (93) protects theunderlying TiW except at the film edges where some undercutting occurs.Because the TiW is only 1700 Å thick, the undercut can be held to 2550 Åeven with a 50% overetch. FIG. 79 illustrates the patterned first levelmetal contacts 7905-7990.

(96) Deposit 2.3 μm thick interlevel oxide 8010 by CVD from reaction ofTEOS and oxygen at 390 C. to cover the first level metal, NiCr resistors7890 and BPSG 7610. The interlevel oxide will provide the interleveldielectric between the first and second metal levels, but the uppersurface of the oxide has topography roughly reflecting the bumpiness ofthe underlying first level metal which has 8000 Å high dropoffs.

(97) Sinter at 450 C. in forming gas (90% N₂ plus 10% H₂); this reducescontact resistance of the first level metal to PtSi to silicon.

(98) Spin on 1.5 μm thick photoresist and expose and develop it todefine locations over the first level metal for vias to second levelmetal. Note that the photoresist has covered the bumpiness of theinterlevel dielectric but has an essentially flat top surface except forthe patterned vias.

(99) Plasma etch the interlevel dielectric 8010 with the via patternedphotoresist as the etch mask using CHF₃ plus O₂ which etches both theinterlevel dielectric and the photoresist. Thus the via pattern persiststhrough the interlevel dielectric and the planar surface of thepatterned photoresist propagates to planarize the interlevel dielectricsurface; however, the isotropic nature of the etch broadens the vias andslopes their sidewalls. The etch stops in the vias when it reaches firstlevel metal except for the lateral etching; thus the depths of the viascan vary to accommodate variation in the thickness of the interleveldielectric. The etch is timed and stopped to insure a minimum thicknessof at least 0.5 μm of interlevel dielectric at its thinnest portion,which occurs over the poly-to-poly capacitors due to the stackedpolysilicon layers. FIG. 80 shows vias stacked over contacts forconvenience, but, in general, vias are not stacked, but displaced to thesides of contacts.

(100) Strip any remaining patterned photoresist with organic solvent.

(101) Sputter deposit a 1.6 μm thick layer 8020 of silicon copperaluminum (about 1% Cu, 0.5% Si) on the planarized interlevel dielectric8010 for second level metal. A back sputter may be required to clean theM1 surface prior to M2 deposition. The second level metal covers thesloped sidewalls of the vias in the interlevel dielectric to connect tothe first level metal exposed at the bottoms of the vias. Bond pads areformed in second level metal.

(102) Spin on photoresist and expose and develop it to define the secondlevel metal interconnections.

(103) Plasma etch the silicon aluminum 8020 with Cl₂ plus BCl₃ and CHF₃using the patterned photoresist as the etch mask.

(104) Strip the patterned photoresist with a plasma of oxygen plus afollow up wet strip with organic solvent as a clean up.

(105) Deposit 0.8 μm thick oxide 8030 by reacting TEOS plus oxygen withthe middle 0.6 μm doped with phosphorus. Then deposit 0.4 μm thicksilicon nitride 8040. The oxide and nitride will form the passivationlayer, which has a total thickness, including the interlevel oxide fromstep (96), of about 2.2 μm over NiCr resistors 7890. A plasma reactionof silane with ammonia and nitrogen using dual RF (13 MHz and 600 KHz)deposits nitride with a low hydrogen and low Si--H bond content toimprove subsequent laser trimming results. Typical nitride films have30% (atomic percent measured) total hydrogen and 20% Si--H bondedhydrogen, whereas nitride 8040 has only 20% total hydrogen and 12% Si--Hbonded hydrogen. Hydrogen not Si--H bonded is usually N--H bonded andstable, so the reduction of Si--H bonded hydrogen by about a factor oftwo provides laser trimming benefits noted in the following. See FIG. 80showing interlevel oxide 8010, second level metal 8020, passivationoxide 8030, and passivation nitride 8040.

(106) Sinter at 450 C. in a forming gas atmosphere to reduce the viaresistance.

(107) Spin on photoresist and expose and develop it to define openingsto the bond pads.

(108) Plasma etch nitride 8040 with CF₄ and wet etch oxide 8030 withbuffered HF down to the bond pads.

(109) Strip the patterned photoresist with organic solvent. Thiscompletes the semiconductor processing of wafer 6001.

To finish the fabrication: probe die on the wafer; laser trim precisionanalog circuits including laser trim the NiCr resistors by focussing alaser beam through interlevel oxide 8010, passivation oxide 8030, andnitride 8040 to vaporize portions of the NiCr film; saw wafer 6001 intodice; mount the individual dice on lead frames; connect bond wires tothe bond pads; electrically test the mounted and bonded dice; and lastlypackage the trimmed dice.

The laser trimming of NiCr thin film resistors typically has a pulsedlaser spot scan the thin film and melt/disperse away portions of it intothe oxide. This increases the resistance by removing metal. However, thekerf area at the edges of the cut portions is a complex scallopedstructure of partially removed metal, and this kerf area can apparentlychange conductivity over time. Indeed, resistors which have been lasertrimmed typically show much greater resistance drift over time thanuntrimmed resistors. Experimentally, laser trimmed NiCr resistors withoxide plus nitride passivation show very good stability when the nitridehas a low Si--H content (12%) and low stress (2×10⁸ dynes/cm²compressive) as in step (105) but poor stability when the nitride hasthe typical high Si--H content (20%) and high stress (2×10⁹ dynes/cm²).There are three possible explanations for the dependence of resistancedrift on the nitride characteristics:

(1) Si--H bonds are weak and nitride with high Si--H content may releasefree hydrogen. Such free hydrogen may assist the regrowth or annealingof kerf areas over the operating life of the resistor and thereby lowerresistance over time. Note that hydrogen has been found responsible forchanging the resistance of single crystal and polysilicon diffusedresistors, and hydrogen in the form of steam has been shown to be moreeffective than oxygen treatment in annealing oxides.

(2) The compressive stress of the nitride may mechanically move the NiCrover time and thereby change its resistance. And the movement in thekerf area will dominate the resistance change.

(3) Si--H bonds scatter the laser light and spread out the spot duringtrimming. This leads to a broader kerf area and consequent greater kerfchanges.

200 C accelerated life testing with the preferred embodiment trimmedNICr resistors gave a resistance drift of only 0±0.2%.

Electrostatic discharge (ESD) protection for integrated circuitsfabricated with the first preferred embodiment method appears in FIGS.82-85. Generally, MOS IC products are prone to ESD damage if their inputand output pins are left unprotected. It is, therefore, a commonpractice to place ESD protection devices between the input or outputpins and the supply voltage rails. MOS diodes as the ESD protectiondevices are used in typical CMOS digital integrated circuits where theoutput signal swings between Vcc (+5.0 V) and Gnd (0 V). Usually, thesilicon substrate is tied to Gnd, which makes the substrate the naturalreturn point for both the output signal and the ESD current.

In mixed mode analog-digital system applications, it is very common tohave blocks of circuits operating from several different voltage supplyrails. For example, a digital circuit based on positive logic levelswould operate in between Vcc (+5 V) and Gnd (0 V), while another digitalcircuit based on unusual negative logic levels operates between Gnd andVee (-5 V). And yet another analog circuit may operate between Vcc andVee. It is rather common for a BiCMOS integrated circuit to have analogcircuits at one end that operate between the full supply rails (Vcc andVee) and digital circuits at another end operating between full supplyrails, or the positive supply rails (Vcc and Gnd) for positive logiccompatibility. In this latter case, all digital signals would return tothe Gnd supply, and therefore the ESD protection for the signal pinswould be implemented conventionally as shown in FIG. 81. It may not benecessary in many cases to place any ESD protection devices between thesignal terminal and the Vee supply rail because it is not relevant tothe operation of the circuit under consideration, especially when gateoxides are relatively thick (greater than 250 Å). Notice, however, thatthe return path for the ESD current is not necessarily the Gnd supplyline in this case, because the silicon substrate is now tied to the Veerail, not the Gnd line, as in MOS. Therefore, the conventional ESDprotection scheme illustrated in FIG. 81 may be vulnerable to the ESDevents when the discharge current path finds its way to the siliconsubstrate, especially for thin gate oxide devices.

In high-performance, high-speed, mixed-mode BiCMOS products, the digitalcircuits would require a thin gate CMOS part, which would exacerbate theaforementioned ESD vulnerability.

The preferred embodiment provides an additional current path between thecircuit terminal and the silicon substrate, even though the actualsignal swings only between the positive supply rails under normaloperating conditions. Since many ESD events occur during integratedcircuit handling by human beings, it is likely that the ESD pulsesdischarge to the silicon substrate rather than to the ground pins, whichwould result in ruptured CMOS gates in the BiCMOS digital circuitry. TheESD protection circuit can be implemented by placing conventional ESDdevices, such as a bipolar transistor operating in BVceo mode, betweenthe circuit terminal and the Vee line.

Two arrangements are shown in FIGS. 82-83. The first one shown in FIG.82 has the ESD protection devices connected directly between the digitalsignal terminal (output in the figures) and the Vee rail in addition tothe conventional ESD protection scheme as shown in FIG. 81 in order toprovide a direct ESD current path to protect the BiCMOS digital circuit.

The second one in FIG. 83 has an ESD device connected between Gnd andVee in addition to the conventional ESD circuit shown in FIG. 81. Inthis circuit, the ESD current would flow through the ESD Device 2 andthe ESD Device 3 in order to provide yet another circuit path to the ESDpulse current.

The actual ESD devices could be any nonlinear devices which present veryhigh impedance to the circuit under normal circuit operation but turn oninto a very low impedance mode when the signal terminal reaches acertain threshold above the normal operating voltage. The ESD capturethreshold of the ESD device should be set in such a way that it ishigher than the normal supply rail voltages but sufficiently lower thanthe gate rupture voltage of the CMOS devices in the BiCMOS digitalcircuit.

FIG. 84 shows an example of the first arrangement: a combination of thebipolar transistor operating in BVceo mode with a bipolar transistordiode with the base shorted to the collector in order to meet the ESDcapture threshold requirements stated above. FIG. 85 shows an example ofthe second arrangement. Here a bipolar transistor operating in BVebsmode is used to meet the ESD threshold requirements.

Noise suppression for integrated circuits fabricated with the firstpreferred embodiment method may be enhanced with the isolation betweendigital and analog regions as shown in FIGS. 86-87. In effect twoparallel buried P+ layers 8601-8602, each about 20 μm wide, andintervening 10 μm buried N+ layer 8605 at a distance of 6 μm from eachP+ buried layer form a moat between digital and analog regions. P+8601-8602 biased to -5 volts and N+ 8605 biased +5 volts sets up a(weak) electric field in the underlying nondepleted substrate 6001 thatintercepts drifting minority electrons injected by the digital devices.Both P+ 8601 and P+ 8602 are needed to create the (symmetrical) fieldwhich penetrates somewhat down into substrate 6001. Opposite polaritieswould also work.

Converter 300 may be fabricated with the first preferred embodimentBICMOS method to fit on a die of size 5.26 mm by 7.31 mm. See FIG. 44for a plan view.

Further Modifications and Variations

The preferred embodiments, both devices and methods, may be modified inmany ways.

For example, the use of the same flash converter for both conversionscould be replaced by the use of two separate flash converters and aconsequent pipelining effect. FIG. 88 shows converter 8800 having twosample and hold blocks 8801-8802 with sample and hold 8802 essentiallyproviding extended holding of the V_(in) acquired by sample and hold8801 while 8801 acquires the next sample. More explicitly, FIG. 89 is asimplified timing diagram illustrating the operation of converter 8800as follows. Sample and hold 8801 follows (acquires) V_(in) (t) and attime 0 switches to hold V_(in) and flash converter 8811 has beenfollowing the output of sample and hold in the same manner as flashconverter 306 follows sample and hold 304. But in converter 8800 sampleand hold 8802 now also acquires the fixed V_(in) being held by sampleand hold 8801. Thus, when error amplifier 8822 needs the V_(in) tocompare to the reconstruction by DAC 8820, sample and hold 8802 willsupply it and sample and hold 8801 can be acquiring the next sample.After 30 nsec of settling, flash converter 8811 latches and after 28nsec supplies the 7 bits to MSB Latch 8830 and then to DAC 8820 in thesame manner as in manner as with converter 300. Sample and hold 8802 nowis holding V_(in) and sample and hold 8801 is released for the nextsample. Error amplifier 8822 operates in the manner of error amplifier312 and flash converter 8812 is following the output of error amplifier8822. Error correction 8834 is analogous to error correction block 318.As soon as the 7 bits from MSB Latch 8830 are put into block 8834,sample and hold 8801 switches again to hold, and flash converter 8811processes the next sample and loads MSB Latch 8830. Thus the overallconversion rate increases by the diminished acquire time required by theinput sample and hold but at the cost of requiring two matched flashconverters and a second sample and hold (which only has to acquire dcsignals).

FIG. 90 illustrates another approach with two sample and hold blocksbeing used to diminish the acquire time: sample and hold blocks 9001 and9002 are ping-ponged to alternately play the role of sample and hold304. The advantage is as with converter 8800: while one sample and holdis holding V_(in) for the error amplifier the other sample and hold isalready acquiring the next sample. Sample and hold blocks 9001-9002 pluscontrolling ping-pong signal could be used directly in place of sampleand hold 304 in converter 300. FIG. 91 shows a timing diagram for theping-pong operation.

The timing controller 4500 could be composed of oscillator cells 4600connected in parallel with differing time delays and with logicalcombinations of the outputs to create the desired timing pulses. Forexample, FIG. 92 shows timing generator 9200 made of four parallel cells4600 with increasing time delays as shown in the top panels of timingdiagram FIG. 93. The logic gates convert the cell outputs to the outputsshown the bottom panels of FIG. 93. Because all of the cells startcharging their timing capacitors when CNTRL goes low, smaller currentsfor the longer time delays can be used, giving lower power consumption.Also, the capacitors could all be the same size and differing chargingcurrents could be obtained by differing mirror device sizes.

Power up reset circuit 5000 could have the NPN 5050 plus NPN diode chain5051 replaced with NMOS versions or even a single NMOS biased at about-2 volts by a resistive divider from ground to Vee. Alternatively,circuit 5000 could have all of the MOS devices, including the invertersand gate, replaced with digital bipolar devices.

The voltage reference 326 could be a bandgap generator with a curvaturecorrection circuit as shown in simplified version in FIG. 94. Indeed,reference circuit 9400 has the standard bandgap reference opamp 9402 andNPNs 9411 and 9431 of different sizes; output resistors 9451-9453 againboost output and provide for the curvature correction current togenerate a temperature dependent voltage increase as with reference 326.The correction circuit in 9400 uses two PMOS differential pairs9482-9483 and 9492-9493 in place of the single NPN differential pair5601-5602 of FIG. 56. An approximately temperature independent bias(Vout/K) drives one PMOS of each pair and a temperature varying bias(the collector of NPN 9431) drives the other PMOS of each pair. Eachpair has a current mirror load but with oppositely driven outputs; thatis, the output of the 9482-9483 pair taps the drain of thetemperature-independently driven PMOS 9482 and the output of the9492-9493 pair taps the drain of the temperature-dependently driven PMOS9493. As with the correction circuit of FIG. 56, diode connections tothe outputs provide the compensation current Icom so no switches need bethrown to provide positive Icom for temperatures both above and belowT_(P).

Error amplifier 312 can be generally used as a two channel amplifierwith different input characteristics: one channel a high input impedanceMOS and the other channel a high gain NPN. Switching between channelsfollows from the control of the bias currents. More generally, multiplechannels could be used with a selection of bias currents as to thecharacteristics desired: two or more channels could have MOS or othertype FET, e.g., JFET, inputs with different device sizes for differentgains or even differing numbers of internal gain stages, two of morechannels could have bipolar inputs (NPN or PNP), and differing gainscould be used for switching between large and small input signals.

As with sample and hold 304, PNP devices could be used in addition tothe NPN and CMOS devices actually appearing in the schematic diagrams.For example, in the output buffer 320 the drivers of FIGS. 41 and 43could have NPNs 4102 and 4302 replaced by PNPs to give a complementaryoutput.

Circuits using structures such as NPN diodes could also be made with MOSdiodes, diodes with resistors, or devices controlled by a fixed bias (ora fraction of a supply rail voltage in the case of start up circuits).

Various processes such as metal, polysilicon or polycide gate, triplelevel metal, silicon-on-insulator, and so forth could be used. P-typeregions and devices can be interchanged with N-types. Indeed, FIG. 95shows a cross sectional elevation view of devices made according to thepreferred embodiment method of fabrication modified for buried oxidesubstrate 9501. Substrate 9501 contains buried oxide layer 9503, whichcan be either implanted or created by bonding wafers or by otherdielectric isolation techniques; the substrate below oxide 9503 need notbe monocrystalline and may even be insulator as in substrate 9501 beingsilicon-on-sapphire. Deep trenches 9505 penetrate to buried oxide 9503to isolate subcircuits, not every device, and extends the idea ofpseudosubstrate 6060 for digital CMOS. This effectively isolates thedigital noise from the analog circuits. A modified fabrication methodwould proceed as follows: start with a substrate having 2-5 μm ofsilicon over an oxide layer; then implant buried layers and grow anepitaxial layer as in the first preferred embodiment. Etch deep trenchesand refill them (with dielectric, oxide/polysilicon, etc.) andplanarize. Then continue as with the first preferred embodiment. Forgreater density seal the shallow trench sidewalls (ROI in firstpreferred embodiment) for prevention of lateral encroachment during theisolation oxidation (e.g., SWAMI type process or polybuffered LOCOS).

Other variations of the first preferred embodiment fabrication methodinclude separate digital and analog power supplies and grounds fordifferent voltage ranges for the digital and analog with correspondingdifferent gate oxide thicknesses, drain doping levels, epilayerthicknesses, and so forth. FIG. 96 illustrates the general split betweendigital and analog circuits where the digital circuits are isolated bythe pseudosubstrate and the analog voltage V need not equal the +5 voltsdigital power supply. Indeed, the effective separation of good digitaland good analog devices permits integration of low noise analog frontends with a significant amount of digital logic to create monolithicitems. Examples of such integration include a complete radio with an RFfront end plus an audio back end, and a video processor with a front endcorrelator followed by analog signal processing (filter, modulator,demodulator, limiter) to provide both an analog signal with the baseband stripped out and a detection of the carrier which is followed byanalog-to-digital conversion of the analog signal plus digital signalprocessing with the carrier detection providing the timing. In fact, theultrasound example of FIG. 1 could have a DSP, a digital-to-analogconverter, and the ultrasound head integrated on a single chip so thatthe transmitted waves could be digitally controlled but drive a highvoltage transducer.

Variations in the first preferred embodiment to enhance the PNPperformance include: replacing steps (69)-(70) which implant boron at100 KeV through 2500 Å thick oxide for PMOS source/drains and PNPemitters with steps immediately following step (62) that will implantboron at 30 KeV through just the mesa oxide and will yield better PNPemitters but shorten the effective channel lengths of the PMOS. Further,less variability in PNP base parameters can be had by an implantseparate from the drain extension implant of steps (52)-(53). Thisseparate PNP base implant would follow step (54) and include phosphorusat 180 KeV with a dose of 1×10¹⁴ ions/cm². The resultant isolated PNPshould have a beta of 60, an Early voltage of 15 volts, f_(T) of 1.3GHz, and breakdown BV_(ceo) of at least 10 volts. Additional enhancementto the Early voltage of the isolated PNP can come from variations thatreduce diffusions, such as dropping step (35) and using quicker gateoxidations.

Another variation providing a PNP which more accurately complements theNPN uses a washed emitter. In particular, P+ emitter implants 7524 and7530 from step (70) are replaced by a separate implant using theapertured BPSG for alignment as with the NPN emitter implant. The PNPemitter implant follows the NPN emitter implant and cap oxidedeposition. The use of washed emitters for the PNP permits the samehigher density as with the NPNs and also probably increases the Earlyvoltage of the PNPs up to 20 volts.

The use of the second polysilicon layer 6950 in place of the firstpolysilicon layer 6710 for the simultaneous doping of polysilicon andsubstrate has an advantage that less thermal oxide is grown on thelocations of the substrate doping because the second gate oxide would begrown prior to second polysilicon layer 6950 deposition. This makes theapertures through the oxide easier to etch. That is, steps (38)-(41)could be moved to follow step (47) with the change that the 185 Å firstgate oxide would now be 300 Å second gate oxide. The advantage of usingfirst polysilicon layer for the simultaneous doping is the further heattreatment to diffuse in the phosphorus.

The use of thinner polysilicon for the CMOS gates would permit the useof thinner BPSG. In this case the ratio of the NPN emitter depth to thetotal oxide thickness etched to form the emitter apertures could be aslow as about 1 to 2.

As with the error amplifier, the voltage follower could be used in ageneral setting with differential inputs and differential outputs. Inparticular, follower B in FIG. 36 could have an output and currentsource matching that of follower A. The load NPNs could be eliminated.

What is claimed is:
 1. A differential voltage follower, comprising:(a) afirst input node connected to a first load; (b) a second input nodeconnected to a second load, said second load matched to said first load;(c) a first follower with input connected to said first input node andwith output connected to a control input of both said first and secondloads; (d) a second follower with input connected to said second inputnode and with output connected to an output node; (e) wherein said firstfollower includes a first current mirror and said second followerincludes a second current mirror, said first and second current mirrorscross-coupled to tie the currents through said followers.
 2. Thedifferential voltage follower of claim 1, wherein:(a) said firstfollower includes a first field effect transistor (FET) with gateconnected to said first input node and source connected to said firstmirror; (b) said second follower includes a second FET with gateconnected to said second input node and source connected to said secondmirror; (c) said first mirror includes a first pair of bipolartransistors (BJTs), the emitters of said first pair of BJTs connectedtogether and forming said first follower output, the bases of said firstpair of BJTs connected together, the collector of one of said first pairof BJTs connected to both its base and the source of said first FET, andthe collector of the other of said first pair of BJTs connected to thesource of said second FET; and (d) said second mirror includes a secondpair of bipolar transistors (BJTs), the emitters of said second pair ofBJTs connected together and forming said second follower output, thebases of said second pair of BJTs connected together, the collector ofone of said second pair of BJTs connected to both its base and thesource of said second FET, and the collector of the other of said secondpair of BJTs connected to the source of said first FET.
 3. Thedifferential voltage follower of claim 2, wherein:(a) said FETs areNMOS; (b) said BJTs are NPN; and (c) said first and second loads areeach an NPN with said control input the base.
 4. The differentialvoltage follower of claim 2, further comprising:(a) a first currentsource connected to said output node; (b) a third current mirror withreference current the current through said second FET and with outputcurrent a current through said first follower.
 5. The differentialvoltage follower of claim 1, wherein:(a) said first follower includes afirst field effect transistor (FET) with gate connected to said firstinput node and source connected to said first mirror; (b) said secondfollower includes a second FET with gate connected to said second inputnode and source connected to said second mirror; (c) said first mirrorincludes a first pair of FETs, the sources of said first pair of FETsconnected together and forming said first follower output, the gates ofsaid first pair of FETs connected together, the drain of one of saidfirst pair of FETs connected to both its gate and the source of saidfirst FET, and the drain of the other of said first pair of FETsconnected to the source of said second FET; and (d) said second mirrorincludes a second pair of FETs, the sources of said second pair of FETsconnected together and forming said second follower output, the gates ofsaid second pair of FETs connected together, the drain of one of saidsecond pair of FETs connected to both its gate and the source of saidsecond FET, and the drain of the other of said second pair of FETsconnected to the source of said first FET.
 6. A differential voltagefollower, comprising:(a) first and second input nodes; (b) first andsecond field effect transistors with a gate of said first transistorcoupled to said first node and a gate of said second transistor coupledto said second node; (c) third, fourth, fifth, and sixth bipolartransistors cross-coupled with the collectors of said third and sixthbipolars connected to the source of said first field effect transistorand the collectors of said fourth and fifth bipolars connected to thesource of said second field effect transistor, said fourth bipolarmirroring the collector current of said third bipolar, and said sixthbipolar mirroring the collector current of said fifth bipolar, wherebythe drain current of said first and second field effect transistors iseach split between said mirroring bipolars; and (d) first and secondoutput nodes, said first output node connected to the emitter of saidthird bipolar and said second output node connected to the emitter ofsaid fifth bipolar.